Extreme ultraviolet light source device, laser light source device for extreme ultraviolet light source device and method for controlling saturable absorber used in extreme ultraviolet light source device

ABSTRACT

An EUV light source of the present invention is capable of using a saturable absorber stably and continuously in a high heat load state. A saturable absorber (SA) device is disposed on a laser beam line to absorb feeble light, such as self-excited oscillation light, parasitic oscillation light or return light. SA gas from an SA gas cylinder and buffer gas from a buffer gas cylinder are mixed to be a mixed gas. The mixed gas is supplied to an SA gas cell via a supply pipeline, and absorbs the feeble light included in the laser beam. The mixed gas is exhausted via an exhaust pipeline, and is sent to a heat exchanger. The mixed gas, cooled down by a heat exchanger, is sent back to the SA gas cell by a circulation pump.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an extreme ultraviolet light sourcedevice, a laser light source device for the extreme ultraviolet lightsource device, and a method for adjusting the laser light source devicefor an extreme ultraviolet light source device.

2. Description of the Related Art

A semiconductor chip is created, for example, by reduction projection ofa mask, on which a circuit pattern is drawn, onto a wafer coated withresist, and repeating such processing as etching and thin filmdeposition. As semiconductor processing increases the degree ofminiaturization, light with an even shorter wavelength is demanded.

To meet this demand, a semiconductor exposure technology for using lightwith an extremely short wavelength, 13.5 nm, and a reduction opticalsystem, is under consideration. This technology is called EUVL (ExtremeUltraviolet Lithography). Hereafter extreme ultraviolet light isreferred to as “EUV light”.

As the EUV light source, an LPP (Laser Produced Plasma) light source, aDPP (Discharge Produced Plasma) light source and an SR (SynchrotronRadiation) light source are known.

The LPP light source is a light source which generates plasma byirradiating a laser beam onto a target material, and using the EUV lightradiated from this plasma. The DPP light source is a laser source whichuses plasma generated by discharge. And the SR light source is a lightsource using orbital radiation light. Among these three types of lightsources, the LPP light source has a high possibility to provide highoutput EUV light, since the plasma density can be increased and a solidangle for collection can be increased more than the other types.

In order to obtain a high output driver laser beam at a high repetitionrate, a laser light source device constituted based on an MOPA (MasterOscillator Power Amplifier) system has been proposed (Japanese PatentApplication Laid-Open No. 2006-128157).

In the LPP light source, a saturable absorber can be used so that alaser beam, reflected by the target material in the chamber and returnedto the optical path (so called “return light”), parasitic oscillationlight and self-excited oscillation light in the amplifier are absorbed.

The saturable absorber has a characteristic to absorb a laser beamhaving an intensity less than a predetermined value. By using asaturable absorber, damage to an amplifier and laser oscillator can beprevented, and the quality of a laser beam can be increased by removingsmall pulses called “pedestals”. A technology on a saturable absorber isknown, however which is not a prior art on an extreme ultraviolet lightsource (U.S. Pat. No. 3,638,137).

For an extreme ultraviolet light source device, a carbon dioxide laser(hereafter CO₂ laser) is used at high output (pulse energy 100 to 200mJ) at high repetition rate (10 to 200 kHz). An extreme ultravioletlight source device is demanded to have a capability to supply a laserbeam having stable pulse energy and pulse waveforms for a long time.

In order to use a CO₂ pulse laser stably for a long time in such a highload state (state where high repetition rate and high pulse energy arerequired), oscillation due to parasitic oscillation, self-excitedoscillation and return light must be suppressed. A possible method is todispose a saturable absorber on the optical path of the laser beam inorder to prevent the parasitic oscillation and self-excited oscillationso that the pulse energy and pulse waveform are stabilized.

In the prior art (U.S. Pat. No. 3,638,137), however, an object of theart is not using a saturable absorber in order to stably use a highoutput CO₂ pulse laser for a long time. Therefore in this prior art, agas cell is disposed in a resonator of the CO₂ laser, and a cylinder ofSF₆ (sulfur hexafluoride) and a cylinder of C₂F₃ are connected to thegas cell, so as to supply a mixed gas (saturable absorber gas) of SF₆and C₂F₃ to the gas cell. The mixed gas supplied to the gas cell absorbsa laser beam less than a predetermined value emits heat, and then isexhausted from the gas cell.

In the prior art, parasitic oscillation, self-excited oscillation oroscillation due to return light can be prevented by supplying mixed gasto the gas cell until gas in each cylinder is emptied. However theamount of gas that can be stored in a gas cylinder is limited, and themixed gas that is once used is exhausted, so gas in each gas cylinderwill eventually be used up. Once each gas cylinder is emptied and can nolonger supply mixed gas to the gas cell, the gas cell cannot normallyperform the expected function.

It is also possible to seal the mixed gas in the gas cell, or todecrease the flow rate of the mixed gas supplied to the gas cell. Inthese cases, however, the temperature of the mixed gas in the gas cellrises when the laser beam passes through, and the mixed gas decomposesand can no longer play the role of a saturable absorber. In this way,the problem of the prior art is that the saturable absorber gas cellcannot be operated stably for a long time.

SUMMARY OF THE INVENTION

With the foregoing in view, it is an object of the present invention toprovide an extreme ultraviolet light source device, a laser light sourcedevice for the extreme ultraviolet light source device and an extremeultraviolet light source device, which can stably operate the saturableabsorber cell for a long time and a method for controlling the saturableabsorber used thereof. It is another object of the present invention toprovide: an extreme ultraviolet light source device, a laser lightsource device for the extreme ultraviolet light source device and anextreme ultraviolet light source device, which can stably operate thesaturable absorber cell for a long time, and further which can correctthe direction and wavefront profile of the laser beam to be apredetermined direction and predetermined shape of wavefront profile byadjusting the wavefront of the laser beam that passes through thesaturable absorber cell to be axially symmetric; and a device and methodfor controlling or stabilizing the saturable absorber used thereof.Other objects of the present invention will be clarified by descriptionof the embodiments herein below.

To solve the above problems, an extreme ultraviolet light source deviceaccording to a first aspect of the present invention is an extremeultraviolet light source device for generating extreme ultravioletlight, the device including: a target material supply unit for supplyingtarget material into a chamber; a laser oscillator for outputting apulse laser beam; at least two amplifiers for amplifying the laser beamthat is output from the laser oscillator; a focusing optical system forirradiating the laser beam onto the target material by focusing thelaser beam, which is amplified by the amplifier, to a predeterminedposition in the chamber; and a saturable absorber device, disposed on anoptical path between the laser oscillator and the predeterminedposition, for absorbing at least a laser beam having light intensity notgreater than a predetermined value and suppressing laser beamtransmission. The saturable absorber device includes: a saturableabsorber cell having an input window that is disposed in an input sidefor the laser beam to enter, an output window that is disposed in anoutput side for the laser beam to be output, a flow space that is formedbetween the windows where the saturable absorber flows, an inlet forletting the saturable absorber enter the flow space, and an outlet forletting the saturable absorber out from the flow space; a pipeline forconnecting the inlet and the outlet; a transport unit, disposed in themiddle of the pipeline, for transporting the saturable absorber thatflows out of the outlet, so as to flow into the flow space via theinlet; and a temperature adjustment unit, disposed in the middle of thepipeline, for adjusting temperature of the saturable absorbertransported by the transporting unit.

According to a second aspect, in the first aspect, the saturableabsorber cell includes: an input window that is disposed on an inputside where the output laser beam of the laser oscillator enters; anoutput window that is disposed on an output side where the output laserbeam of the laser oscillator is output; the flow space formed betweenthe windows; the inlet; and the outlet.

According to the third aspect, in the first aspect, the inlet and theoutlet are disposed so that the flow of the saturable absorber in theflow space becomes approximately symmetric with respect to an opticalaxis of the laser beam that passes through the flow space.

According to the fourth aspect, in the first aspect, the inlet and theoutlet are disposed so that the flow of the saturable absorber in theflow space becomes approximately symmetric with respect to an opticalaxis of the laser beam that passes between the windows.

According to the fifth aspect, in the second aspect, the inlet or theoutlet is disposed near each window, so that the saturable absorberflows on an inner face side of each of the windows and the saturableabsorber moves along the optical axis of the laser beam between thewindows.

According to the sixth aspect, in the second aspect, the inlet isdisposed on the input window side and the output window siderespectively, and the outlet is disposed between the windows.

According to the seventh aspect, in the second aspect, the outlet isdisposed on the input window side and the output window siderespectively, and the inlet is disposed approximately at the centerbetween the windows.

According to the eighth aspect, in the sixth aspect, the inlet isdisposed inclined toward the center of the inner face of each window.

According to the ninth aspect, in the seventh aspect, the outlet isdisposed inclined toward the center of the inner face of each window.

According to the tenth aspect, in the second aspect, each window isformed to be circular, a plurality of inlets are disposed on the inputwindow side and the output window side respectively, so as to be axiallysymmetric with respect to the optical axis of the laser beam that passesbetween the windows, each inlet disposed on the input window side isdisposed in parallel with a tangential line direction of the inputwindow, and each inlet disposed on the output window side is disposed inparallel with a tangential line direction of the output window, and aplurality of outlets are disposed between the windows so as to beaxially symmetric with respect to the optical axis of the laser beam.

According to the eleventh aspect, in the second aspect, each windows isformed to be circular, a plurality of outlets are disposed on the inputwindow side and the output window side respectively, so as to besymmetric with respect to the laser beam passing between the windows,each outlets disposed on the input window side is disposed in parallelwith a tangential line direction of the input window, and each outletdisposed on the output windows side is disposed in parallel with atangential line direction of the output window, and a plurality ofinlets are disposed between the windows so as to be symmetric withrespect to the laser beam.

According to the twelfth aspect, in the second aspect, each window isformed to be circular, a plurality of outlets are disposed on the inputwindow side and the output window side respectively, so as to be axiallysymmetric with respect to the optical axis of the laser beam that passesbetween the windows, and a plurality of inlets are disposed between thewindows so as to be axially symmetric with respect to the optical axisof the laser beam, and a flow control member, in which a plurality offlow holes for letting the saturable absorber flow are formed, isdisposed between an outer circumference of the inner face side of eachwindow and each outlet.

According to the thirteenth aspect, in the twelfth aspect, the flowcontrol member comprises: a tubular member which is disposed coaxiallyin each window, and one edge of which is disposed on the inner face sideof each window, and which has the flow hole individually; and aring-shaped collar portion that covers an area between the other edge ofthe tubular member and an inner wall portion of the flow space, so thatthe saturable absorber does not flow into each outlet from an area otherthan each flow hole.

According to the fourteenth aspect, in the first aspect, a plurality ofthe saturable absorber cells are disposed, such that a flow direction ofthe saturation absorber in one saturable absorber cell and a flowdirection of the saturated absorber in the other saturable absorber cellare opposite from each other.

According to the fifteenth aspect, in the first aspect, the inputwindows and the output window are constituted by a common window, and areflection optical system for reflecting the output laser beam of thelaser oscillator that enters from the common window and letting thelaser beam output from the common window is disposed in the flow space.

According to the sixteenth aspect, in any of the first to fifteenthaspects, the extreme ultraviolet light source device further comprises awavefront compensation device for compensating a wavefront of the laserbeam that passes the saturable absorber device.

According to the seventeenth aspect, in the sixteenth aspect, thewavefront compensation device includes: a wavefront measurement unit fordirectly or indirectly measuring a direction and a wavefront profile ofthe laser beam; a wavefront compensation unit for compensating thedirection and the wavefront profile of the laser beam to be apredetermined direction and a predetermined wavefront profile; and awavefront control unit for operating the wavefront compensation unitbased on a measurement result from the wavefront measurement unit.

According to the eighteenth aspect, in the first aspect, the extremeultraviolet light source device further comprises a control unit forcontrolling the transport unit and the temperature adjustment unit.

According to the nineteenth aspect, in the first aspect, the temperatureadjustment unit is constituted by a heat exhausting device forexhausting heat absorbed by the saturable absorber.

According to the twentieth aspect, in any one of the first to sixteenthaspects, the temperature adjustment unit is constituted by a heatexhausting device for exhausting heat absorbed by the saturableabsorber, and a control device for controlling the temperature of thesaturable absorber.

An extreme ultraviolet light source device according to the twenty firstaspect of the present invention is an extreme ultraviolet light sourcedevice for generating an extreme ultraviolet light, the deviceincluding: a target material supply unit for supplying target materialinto a chamber; a laser oscillator for outputting a pulse laser beam; atleast two amplifiers for amplifying the laser beam that is output fromthe laser oscillator; a focusing optical system for irradiating anamplified laser beam onto the target material by focusing the laserbeam, which is amplified by the amplifier and is output, to apredetermined position in the chamber; and a saturable absorber device,disposed on an optical path between the laser oscillator and thepredetermined position, for absorbing at least a laser beam having alight intensity not greater than a predetermined value and suppressinglaser beam transmission, and the saturable absorber device includes asaturable absorber cell having a main body unit that has a flow spacewhere the saturable absorber flows, an inlet for letting the saturableabsorber enter the flow space, an outlet for letting the saturableabsorber out from the flow space, and a window constituted by diamondsfor passing the output laser beam from the laser oscillator to the flowspace.

A pulse laser device according to the present invention includes: alaser oscillator for outputting a pulse laser beam; at least twoamplifiers for amplifying the laser beam that is output from the laseroscillator; and a saturable absorber device, disposed on an optical pathbetween the laser oscillator and the amplifier, or on an optical pathbetween the amplifiers, for absorbing at least a laser beam having lightintensity not greater than a predetermined value and suppressing laserbeam transmission, and the saturable absorber device includes: asaturable absorber cell having an input window that is disposed in aninput side for the laser beam to enter, an output window that isdisposed in an output side for the laser beam to be output, a flow spacethat is formed between the windows where the saturable absorber flows,an inlet for letting the saturable absorber enter into the flow space,and an outlet for letting the saturable absorber out from the flowspace; a pipeline for connecting the inlet and the outlet; a transportunit, disposed in the middle of the pipeline, for transporting thesaturable absorber that flows out of the outlet, so as to flow into theflow space via the inlet; and a temperature adjustment unit, disposed inthe middle of the pipeline, for adjusting temperature of the saturableabsorber transported by the transport unit. The input window and theoutput window may be one common window.

A method for controlling a saturable absorber according to the presentinvention is a method for controlling a saturable absorber that is usedfor an extreme ultraviolet light source device that generates extremeultraviolet light, the method having the steps of: circulating thesaturable absorber in a saturable absorber cell in which a laser beamfor generating the extreme ultraviolet transmits; maintainingtemperature of the saturable absorber to be circulated at apredetermined temperature; and letting the saturable absorber flow inthe saturable absorber cell, so that temperature of distribution of eachwindow of the saturable absorber cell becomes approximately symmetricwith respect to an optical axis of the laser beam.

According to the present invention, the saturable absorber can becirculated and reused, and the temperature of the saturable absorber canbe adjusted. Therefore decomposition of the saturable absorber due toheat can be suppressed, and the saturable absorber can be operatedstably for a long time.

According to the present invention, the inlet and the outlet aredisposed so that the flow of the saturable absorber in the flow spacebecomes approximately symmetric with respect to the optical axis of thelaser beam that passes through between the windows. Therefore thetemperature distribution of each window of the saturable absorber cellcan be axially symmetric with respect to the optical axis of the laserbeam. As a result, the profile of the wavefront of the laser beam thatpasses through the saturable absorber cell can be axially symmetric, andthe wavefront of the laser beam can be easily compensated.

According to the present invention, the window of the saturable absorbercell can be constituted by diamonds. Since the thermal conduction of adiamond window is very high, the temperature distribution that isgenerated on the window can be controlled. Hence the wavefront anddirection of the laser beam that transmits through the diamond windowcan be stabilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an EUV light source device according to afirst embodiment of the present invention;

FIG. 2 is a block diagram of an SA device;

FIG. 3 is a diagram depicting an SA gas cell;

FIG. 4 is a diagram depicting a state of how the wavefront and thedirection of the laser beam that transmits through the saturableabsorber change;

FIG. 5 is a graph depicting the temperature change of the saturableabsorber;

FIG. 6 is a flow chart depicting a general operation of the SA device;

FIG. 7 is a flow chart depicting gas exhaust and gas filling processing;

FIG. 8 is a flowchart depicting processing for preparing the SA gas cellfor use;

FIG. 9 is a flow chart depicting processing for controlling gascirculation and gas temperature;

FIG. 10 is a flow chart depicting processing for detecting abnormality;

FIG. 11 is a diagram depicting configuration of a wavefront compensator;

FIG. 12 is a diagram depicting a sensor;

FIG. 13 is a flow chart depicting a wavefront compensating processing;

FIG. 14 is a flow chart depicting processing when a laser controllernotifies adjustment completion to an EUV light source controller;

FIG. 15 is a diagram depicting configuration of an SA gas cell accordingto the second embodiment;

FIG. 16 is a diagram depicting configuration of an SA gas cell accordingto the third embodiment;

FIG. 17 is a cross-sectional view of a porous cylindrical pipe;

FIG. 18 is a block diagram of an SA device according to the fourthembodiment;

FIG. 19 are diagrams depicting a method for installing the wavefrontcompensator according to the fifth embodiment;

FIG. 20 are diagrams continuing from FIG. 19;

FIG. 21 are diagrams depicting a configuration of a wavefront curvaturecompensator according to the sixth embodiment;

FIG. 22 are diagrams depicting a configuration of a wavefront curvaturecompensator according to the seventh embodiment;

FIG. 23 is a diagram depicting a configuration of a wavefront curvaturecompensator according to the eighth embodiment;

FIG. 24 is a diagram depicting a configuration of a wavefront curvaturecompensator according to the ninth embodiment;

FIG. 25 is a diagram continuing from FIG. 24;

FIG. 26 are diagrams depicting a configuration of a wavefront curvaturecompensator according to the tenth embodiment;

FIG. 27 are diagrams depicting a configuration of a wavefront curvaturecompensator according to the eleventh embodiment;

FIG. 28 are diagrams depicting a configuration of a wavefrontcompensator according to the twelfth embodiment;

FIG. 29 are diagrams depicting a configuration of a wavefrontcompensator according to the thirteenth embodiment;

FIG. 30 are diagrams depicting a configuration of a wavefrontcompensator according to the fourteenth embodiment;

FIG. 31 is a diagram depicting a configuration of a sensor according tothe fifteenth embodiment;

FIG. 32 is a diagram depicting a configuration of a sensor according tothe sixteenth embodiment;

FIG. 33 is a diagram depicting a configuration of a sensor according tothe seventeenth embodiment;

FIG. 34 is a diagram depicting a configuration of a sensor according tothe eighteenth embodiment;

FIG. 35 is a diagram depicting a configuration of a sensor according tothe nineteenth embodiment;

FIG. 36 is a diagram depicting a configuration of a sensor according tothe twentieth embodiment;

FIG. 37 is a diagram depicting a configuration of an SA device accordingto the twenty first embodiment; and

FIG. 38 is a diagram depicting a configuration of an SA device accordingto the twenty second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detailwith reference to the drawings. In the present embodiment, a saturableabsorber device 33 is disposed on an optical path where a laser beampasses, as described below. The saturable absorber device 33 circulatessaturable absorber gas and reuses it while controlling the temperatureof the saturable absorber gas. In the following description, saturableabsorber is abbreviated to SA. Therefore the saturable absorber device33 is called an SA device 33, the saturable absorber gas is called an SAgas, and the saturable absorber gas cell 330 is called an SA gas cell330 respectively.

Embodiment 1

A first embodiment of the present invention will now be described withreference to FIG. 1 to FIG. 14. FIG. 1 is a block diagram depicting ageneral configuration of an EUV light source device 1.

The EUV light source device 1 has, for example, a chamber 10 forgenerating EUV light, a laser light source device 2 for supplying laserlight to the chamber 10, and an EUV light source controller 70. Thelaser light source device 2 has, for example, a laser oscillator (MasterOscillator) 20 that determines the time waveform and repetition rate ofthe laser pulse, an amplification system 30, a focusing system 40, awavefront correction controller (WFC-C) 50, and a laser controller 60.The EUV light source device 1 supplies EUV light to an EUV exposuredevice 5. In the following description and drawings, the wavefrontcompensation controller may be indicated as WFC-C (Wave FrontCompensator Controller).

An overview of the chamber 10 will be described first. The chamber 10has, for example, a chamber main body 11, a connection unit 12, a window13, an EUV collector mirror 14, and a target material supply unit 15.

The chamber main body 11 is kept in a vacuum state by a vacuum pump,which is not illustrated. In the chamber main body 11, a mechanism forcollecting debris, for example, can be installed.

The connection unit 12 is disposed so as to connect the chamber 10 andthe EUV exposure device 5. The EUV light generated in the chamber mainbody 11 is supplied to the EUV exposure device 5 via the connection unit12.

The window 13 is disposed on the chamber main body 11. The driver laserbeam from the laser light source device 2 enters the chamber main body11 via the window 13.

The EUV collector mirror 14 is a mirror for reflecting the EUV light andcollecting it to an intermediate focus (IF). The intermediate focus (IF)is set in the connection unit 12. The EUV collector mirror 14 ideallyhas a concave surface, such as a spheroid, that generates no aberration,in order to transfer and form an image of a plasma generation point atIF. A multilayer coating constituted by molybdenum coating and siliconcoating, for example, is disposed on the surface of the EUV collectormirror 14, whereby the EUV light with about a 13 nm wavelength isreflected.

The target material supply unit 15 supplies the target material, such astin, as a liquid, solid or gas. The tin can also be supplied as a tincompound, such as stannane (SnH₄). If tin is supplied as liquid, amethod for liquidizing pure tin by heating it up to a melting point, ora method for supplying a solution containing tin, or a colloidalsolution containing tin or a tin compound, can be used. In thedescription of the present embodiment, a tin droplet DP is used for anexample of a target material, but the present invention is not limitedto a tin droplet. Another material, such as lithium (Li) or Xenon (Xe),for example, may be used.

Movement of the chamber 10 will be described first in brief. A driverlaser beam L1 focuses at a predetermined position in the chamber mainbody 11 via the input window 13. The target material supply unit 15drops the tin droplet DP onto this predetermined position. Thepredetermined position in this case is a position on a first focal pointof the EUV collector mirror 14. The tin droplet DP is dropped onto thisfirst focal point. The optical system is disposed such that the firstfocal point and the focusing point of the laser match. And at the sametiming as the time when the tin droplet DP reaches the predeterminedposition, the laser light source device 2 outputs the driver laser beamL1 with a predetermined output. The tin droplet DP is irradiated by thefocused driver laser beam L1, and becomes plasma PLZ. The plasma PLZradiates EUV light L2. The EUV light L2 is collected to the intermediatefocus IF in the connection unit 12 by the EUV collector mirror 14, andis supplied to the EUV exposure device 5.

Now the configuration of the laser light source device 2 will bedescribed. The laser light source device 2 is constituted as a carbondioxide pulse laser light source device, and outputs the pulses of thedriver laser beam L1 having a 10.6 μm wavelength, a single lateral mode,a 100 kHz repetition rate, a 100 to 200 mJ pulse energy, and 10 kW to 20kW, for example.

The laser beam that is output from the laser oscillator 20 is amplifiedby the amplification system 30, and is then transmitted to a focusingsystem 40. The focusing system 40 supplies the driver laser beam L1 tothe chamber 10. The focusing system 40 has, for example, a reflectingmirror 41, an off-axis parabolic concave mirror 42, and a relay opticalsystem 43. In the following description, the oscillator 20 side iscalled the “upstream side”, and the chamber 10 side is called the“downstream side” with the laser beam traveling direction as areference.

The amplification system 30 has, for example, a relay optical system 31,a pre-amp (preamplifier) 32, an SA device 33, a wavefront compensator(WFC) 34, a main-amp (main amplifier) 35 and a sensor 36. In thefollowing description and drawings, the preamplifier is referred to asthe “pre-amp, and the main amplifier is referred to as the “main-amp”.The wavefront compensator may be referred to as the WFC (Wave FrontCompensator). The SA device 33 will be described later with reference toFIG. 2 and FIG. 3.

The relay optical system 31 is an optical system for adjusting adivergent angle of a beam and a size of the beam of the laser beam thatis output from the laser oscillator 20, so that the amplification areain the pre-amp 32 is efficiently filled with the laser beam that isoutput from the laser oscillator 20. The relay optical system 31 expandsthe beam diameter of the laser beam that is output from the laseroscillator 20 so as to be transformed to be a predetermined beamluminous flux.

The pre-amp 32 amplifies the entered laser beam, and outputs it. Thelaser beam amplified by the pre-amp 32 enters the SA device 33. The SAdevice 33 has a function to let the laser beam have a light intensitynot less than a predetermined threshold pass, and not to let the laserbeam have a light intensity less than the predetermined threshold pass.

By this, the SA device 33 absorbs the laser beam that returns from thechamber 10 (return light), the parasitic oscillation light and theself-excited oscillation light from the main-amp, so as to preventdamage to the pre-amp 32 and the laser oscillator 20. The SA device 33also plays a role of improving the quality of pulse waveforms of thelaser beam by suppressing pedestals. A pedestal is a small pulse whichis generated in a close proximity time before/after a main pulse.

FIG. 2 is a diagram depicting a configuration of the SA device 33. TheSA device 33 has, for example, an SA gas cell 330, a circulation pump331 (1), an exhaust pump 331 (2), a heat exchanger 332, a supply pipeline 333 (1), an exhaust pipeline 333 (2), an SA gas cylinder 334 (1), abuffer gas cylinder 334 (2), individual valves 335 (1) to 335 (4), apressure sensor 336 (1), a temperature sensor 336 (2), an SA gas cellcontroller 80, and a gas temperature controller 82.

The configuration shown in FIG. 2 is an example, and the presentinvention is not limited to the configuration shown in FIG. 2. Thepresent invention can be implemented only if the device for cooling SAgas (including mixed gas) heated by the laser beam (e.g. heat exchanger)is included in a circuit for circulating the SA gas. In other words, itis sufficient if the configuration allows exhausting the heat of the SAgas while circulating the SA gas. If this configuration is used, the SAgas is not decomposed by heating of the SA gas at least during normaloperation, and a lengthy operation can be possible.

The SA gas cell 330 will be described first with reference to FIG. 3.The SA gas cell 330 has, for example, a tubular holder 3300 with a watercooling jacket, windows 3332 (1) and 3332 (2), which are disposed onboth ends of the holder 3300, a flow space 3334 disposed between thewindows, inlets 3336 (1) and 3336 (2) which are disposed in the holder3300 so as to be connected to a flow space 3334, and outlets 3338 (1)and 3338 (2) which are disposed in the holder 3300 so as to be connectedto the flow space 3334. The holder 3330 is an example of a “main bodyunit having a flow space”.

Each window 3332 (1) and 3332 (2) is formed to be a disk, using amaterial that has high thermal conductivity, and which is transparent toa CO₂ laser beam. An example of such a material is diamond.

A diamond window has very high thermal conductivity, so the influence ofheat conducted from the SA gas on the diamond window can be decreased.Therefore distortion of the wavefront of the laser beam, that passesthrough the SA gas, can be suppressed. In other words, by using adiamond window, distortion of the wavefront can be prevented withoutinstalling a circulation system to circulate the SA gas and atemperature control system to control the temperature of the SA gas.

The laser beam enters from the input window 3332 (1) to the flow space3334, passes through the flow space 3334, and is output from the outputwindow 3332 (2). Each window 3332 (1) and 3332 (2) is installed in theholder 3330 air-tightly via such a seal element as an O-ring.

The flow space 3334 is a tubular space created by each window 3332 (1)and 3332 (2) and the holder 3300. The axis center of the flow space 3334(center line in the lateral direction in FIG. 3) and the optical axis ofthe laser beam L1 are set to be coaxial as default.

In the flow space 3334, a plurality of inlets 3336 (1) and 3336 (2) aredisposed at the input window 3332 (1) side, and a plurality of outlets3338 (1) and 3338 (2) are disposed at the output window 3332 (2) side.Each inlet 3336 (1) and 3336 (2) and each outlet 3338 (1) and 3338 (2)are disposed facing each other to be symmetric with respect to theoptical axis of the laser beam respectively. The supply pipeline 333 (1)is connected to each inlet 3336 (1) and 3336 (2), and the exhaustpipeline 333 (2) is connected to each outlet 3338 (1) and 3338 (2).

The SA gas flows in from the supply pipeline 333 (1) to the flow space3334 via each inlet 3336 (1) and 3336 (2). After flowing in the diameterdirection along the inner face side of the input window 3332 (1), the SAgas moves in the flow space 3334 to the outlet side along the opticalaxis of the laser beam. The SA gas flows along the inner face side ofthe output window 3332 (2) in the diameter direction, and flows intoeach outlet 3338 (1) and 3338 (2). The SA gas is exhausted from eachoutlet 3338 (1) and 3338 (2) via the exhaust pipeline 333 (2), and issent to the heat exchanger 332. The inner face refers to one of thesurfaces of the window that faces the flow space 3334.

Referring back to FIG. 2, the circulation pump 331 (1) is a pump forcirculating the SA gas in the SA device 33. The outlet of thecirculation pump 331 (1) is connected to the supply pipeline 333 (1),and the inlet of the circulation pump 331 (1) is connected to theexhaust pipeline 333 (2) via the heat exchanger 332. The circulationpump 331 (1) is activated according to an instruction from the SA gascell controller 80, and supplies SA gas, cooled down by the heatexchanger 332, to the supply pipeline 333 (1).

The exhaust pump 331 (2) is a pump for removing the SA gas from the SAdevice 33. The exhaust pump 331 (2) is connected to a middle of theexhaust pipeline 333 (2) via the exhaust valve 335 (2). The exhaustvalve 335 (2) can be constituted by a normally-closed two-portelectromagnetic valve, for example. The supply valve 335 (1) and exhaustvalve 335 (2) are activated according to an instruction from the SA gascell controller 80, and suck exhaust SA gas stagnating in each pipeline333 (1) and 333 (2), SA gas cell 330 and heat exchanger 332.

The heat exchanger 332 is disposed such that the supply pipeline 333 (1)and the exhaust pipeline 333 (2) are connected. The heat exchanger 332is a device for keeping the temperature of the SA gas constant. The heatexchanger 332 is activated according to an instruction from the gastemperature controller 82, and adjusts the temperature of the SA gas.

As mentioned above, the supply pipeline 333 (1) is a pipeline forsupplying the SA gas to the SA gas cell 330, and the exhaust pipeline333 (2) is a pipeline for exhausting the SA gas from the SA gas cell330. The supply pipeline 333 (1) and the exhaust pipeline 333 (2) areconnected via the heat exchanger 332 and the circulation pump 331 (1).

The SA gas cylinder 334 (1) is a container for storing SA gas, such asSF₆ (sulfur hexafluoride). The buffer gas cylinder 334 (2) is acontainer for storing buffer gas, such as N₂ (nitrogen) and rare gas(He, Ar). The types of SA gas and buffer gas are not limited to thethose mentioned above.

A desirable SA gas can be selected according to the wavelength of thecarbon dioxide laser. If the wavelength of the carbon dioxide laser beamis 10.6 μm, for example, then SF₆ can be used. If the wavelength of thecarbon dioxide laser beam is 9.6 μm, for example, CH₃OH, CH₃F, HCOOH,CD₃OD, CD₃F and DCOOD (D: deuterium) can be used. And if the wavelengthof the carbon dioxide laser beam is 9.3 μm, for example, then C₂F₂Cl canbe used.

The outlet of the SA gas cylinder 334 (1) is connected to the inlet ofthe gas supply valve 335 (1) via the SA gas valve 335 (3). In the sameway, the outlet of the buffer gas cylinder 334 (2) is connected to theinlet of the gas supply valve 335 (1) via the buffer gas valve 335 (4).The outlet of the gas supply valve 335 (1) is connected to a middle ofthe supply pipeline 333 (1). Each valve 335 (1), 335 (3) and 335 (4) areconstituted by a normally-closed two-port electromagnetic valve, andopens/closes according to an instruction of the SA gas cell controller80.

FIG. 2 shows a case when the SA gas and the buffer gas are provided bydifferent gas cylinders 334 (1) and 334 (2) respectively, and mixed gasis generated in the SA device 33. The present invention is not limitedto this, but a mixed gas cylinder, for storing mixed gas in which the SAgas and the buffer gas are mixed at a predetermined ratio, may be used.In this case, the outlet of the mixed gas cylinder is connected to theinlet of the gas supply valve 335 (1). Therefore the valves 335 (3) and335 (4) in FIG. 2 can be omitted, and a configuration of the SA device33 can be simplified.

The pressure sensor 336 (1) is a sensor for detecting pressure in the SAgas cell 330, and outputting it to the SA gas cell controller 80. Thetemperature sensor 336 (2) is a sensor for detecting the temperature inthe SA gas cell 330, and outputting it to the gas temperature controller82.

The SA gas cell controller 80 is a control device for controllingoperation of the SA device 33. The SA gas cell controller 80 circulatesthe mixed gas of the SA gas and the buffer gas in the SA device 33 byexecuting the later mentioned processing, and controls the temperatureof the mixed gas. The SA gas cell controller 80 notifies the controlresult to the laser controller 60.

The gas temperature controller 82 is a control device for controllingthe heat exchanger 332 so that the temperature of the mixed gas becomesa predetermined temperature. The gas temperature controller 82 controlsthe operation of the heat exchanger 332 according to an instruction fromthe SA gas cell controller 80, and notifies the result to the SA gascell controller 80. The SA gas cell controller 80 may control thetemperature of the mixed gas without differentiating the gas temperaturecontroller 82 and the SA gas cell controller 80.

FIG. 4 shows a state of change of the direction and the wavefrontprofile of the laser beam that passes through the SA gas cell. Theconfiguration of the SA gas cell is simplified in FIG. 4 to makeexplanation easier. Therefore in FIG. 4, the SA gas cell is denoted with330P so as to distinguish it from the SA gas cell 330 in FIG. 3.

FIG. 5 is a graph depicting the temperature distribution generated inthe SA gas cell 330. The mixed gas flows from the inlet 3336 to the flowspace 3334 between the windows 3332 (1) and 3332 (2), absorbs a laserbeam not greater than the threshold of the laser beam, and flows outfrom the outlet 3338. Thereby the temperature distribution which shiftsin the flow direction of the mixed gas is generated in the SA gas cell330. Because of the temperature distribution of the windows 3332 (1) and3332 (2), the distribution of the refractive index of the windows 3332(1) and 3332 (2) change.

As a result, the edge L1 (1) and the edge L1 (2) of the laser beam thatpasses through the SA gas cell 330 shift in a direction AX1 e, whichdeviated from the reference optical axis AX1, as shown in the brokenlines L1 e (1) and L1 e (2) in FIG. 4. The wavefront of the laser beamL1 that passes through the SA gas cell 330 does not changeconcentrically maintaining the reference optical axis AX1 at the center,but shifts along the axis AX1 e. In other words, by passing through theSA gas cell 330, the direction of the laser beam L1 shifts, and thewavefront profile also changes.

Even if the laser beams Lie (1) and Lie (2), of which travelingdirection and wavefront profile shifted, directly enter the main-amp 35,the expected amplification function cannot be implemented. This isbecause the amplification area of the main-amp 35 cannot be efficientlyfilled by the laser beam.

In the present embodiment, to solve this problem, each inlet 3336 (1)and 3336 (2) and each outlet 3338 (1) and 3338 (2) are disposed, asshown in FIG. 3, so that the temperature distribution of each window3332 (1) and 3332 (2) of the SA gas cell 330 become axially symmetricwith respect to the optical axis of the laser beam. Thereby according tothe present embodiment, the wavefront profile of the laser beam thatpasses through the SA gas cell 330 can be axially symmetric with respectto the optical axis of the laser beam. Furthermore according to thepresent embodiment, as shown in FIG. 1 the wavefront compensator (WFC)34 is disposed between the SA gas cell 330 and the main-amp 35. Anexample of the wavefront compensator (WFC) 34 will be described later. Amethod for controlling the wavefront compensator (WFC) 34 according tothe signal from the sensor 36 will also be described later withreference to another diagram.

The operation of the SA device 33 will now be described with referenceto FIG. 6 to FIG. 10. FIG. 6 is a flow chart depicting a main processingthat is a general operation, and FIG. 7 to FIG. 10 are flow chartsdepicting the subroutines called up during main processing. Each flowchart to be described below shows an overview of each processing, whichmay be different for an actual computer program. Those skilled in theart would change or delete a step or add a new step in the flow charts.

In the main processing in FIG. 6, the SA gas cell controller 80 executesgas exhausting and gas filling processing (S10) first. Details on S10will be described in FIG. 7. In S10, the SA device 33 is exhausted byvacuuming and then filled with the mixed gas.

Then the SA gas cell controller 80 executes the processing for preparingthe SA gas cell for use (S11). S11 will be described in detail later inFIG. 8. In S11, the heat exchanger 332 is operated so that thetemperature of the mixed gas that circulates in the SA device 33 becomesa predetermined temperature.

When the SA device 33 is prepared, the laser controller 60 controls thelaser oscillator 20 to emit the laser beam (S12). The SA gas cellcontroller 80 executes processing to circulate gas and control the gastemperature (S13). S13 will be described in detail later in FIG. 9. InS13, inside the SA device 33 is controlled so that the mixed gas at apredetermined temperature circulates.

Then the SA gas cell controller 80 executes the abnormality detectionprocessing (S14). S14 will be described in detail later in FIG. 10. Thenthe SA gas cell controller 80 determines whether an abnormality wasdetected (S15) in the abnormality detection processing (S14).

If there is no abnormality (S15: NO), S12 to S15 are repeated. If anabnormality was detected (S15: YES), the SA gas cell controller 80notifies the laser controller 60 that an abnormality occurred (S16). Thelaser controller that received this notice outputs the signal to thelaser oscillator 20, and stops oscillation of the laser beam (S17).After the abnormality is cleared, the SA gas cell controller 80 returnsto S10, and fills the mixed gas into the SA gas cell 330 again.

FIG. 7 is a flow chart depicting the gas exhausting and gas fillingprocessing (S10 in FIG. 6). The SA gas controller 80 opens the gassupply valve 335 (1) and exhaust valve 335 (2), and closes the SA gasvalve 335 (3) and the buffer gas valve 335 (4) respectively (S20). For avalve constituted by a normally-closed electromagnetic valve, it isunnecessary to output a close valve signal to close the valve from theSA gas cell controller 80.

The SA gas cell controller 80 activates the exhaust pump 331 (2) (S21),and reads the detection signal from the pressure sensor 336 (1) (S22).The SA gas cell controller 80 determines whether the pressure value PSAdetected by the pressure sensor 336 (1) became a predetermined pressurevalue PTh1 or less (S23). The predetermined pressure PTh1 is set to be avalue in a pressure (substantially vacuumed) state that does notnegatively influence the performance of the SA.

If the pressure value PSA drops to a predetermined pressure value PTh1(S23: YES), this means that the old mixed gas in the SA device 33 hasexhausted to the outside. Therefore the SA gas cell controller 80outputs a signal to the exhaust valve 335 (2) to close the valve, andstops the exhaust pump 331 (2) (S24).

The SA gas cell controller 80 outputs the signal to the SA gas valve 335(3) to open the valve (S25). The SA gas stored in the SA gas cylinder334 (1) flows into the supply pipeline 333 (1) via the SA gas valve 335(3) and the gas supply valve 335 (1), and flows into the exhaustpipeline 333 (2) and the heat exchanger 332 via the SA gas cell 330. Asthe SA gas is filled into the SA device 33, the pressure valve PSAdetected by the pressure sensor 336 (1) increases. The SA gas cellcontroller 80 stands by until the pressure value PSA reaches apredetermined SA gas pressure value PSF6 (S26).

If the pressure value PSA reaches the predetermined SA gas pressurevalue PSF6 (S26: YES), this means that the predetermined amount of SAgas has filled the SA device 33, so the SA gas cell controller 80 closesthe SA gas calve 335 (3) (S27).

Then the SA gas cell controller 80 outputs a signal to the buffer gasvalve 335 (4) to open the valve (S28). Thereby the buffer gas stored inthe buffer gas cylinder 334 (2) flows into the supply pipeline 333 (1)via the buffer gas valve 335 (4) and gas supply valve 335 (1). Thebuffer gas also flows into the SA gas cell 330, exhaust pipeline 333 (2)and heat exchanger 332, and is mixed with the SA gas which has beenfilled.

The SA gas cell controller 80 stands by until the pressure valve PSAdetected by the pressure sensor 336 (1) reaches a predetermined mixedgas pressure PMG (S29). If the pressure value PSA reached thepredetermined mixed gas pressure PMG (S29: YES), this means that the SAdevice 33 is filled with the mixed gas at the predetermined pressurePMG, therefore the SA gas cell controller 80 closes the buffer gas valve335 (4) (S30), and also closes the gas supply valve 335 (1) (S31).

FIG. 8 is a flowchart depicting processing for preparing the SA gas cellfor use (S11 in FIG. 6). The SA gas cell controller 80 outputs a signalto the circulation pump 331 (1) for activation (S40). Then the SA gascell controller 80 reads the temperature TSA that is detected by thetemperature sensor 336 (2) (S41). The SA gas cell controller 80calculates the deviation ΔT between the detected temperature TSA and apredetermined reference temperature Tset (S42).

The SA gas cell controller 80 determines whether the absolute value ofthe calculated deviation ΔT is a predetermined threshold Tth or less(S43). If the deviation ΔT is the predetermined threshold Tth or less(S43: YES), it is determined that the temperature of the mixed gas isbeing kept constant, and the SA gas cell controller 80 outputs apreparation completion signal of the laser controller 60 (S44). Thepreparation completion signal is a signal to indicate that thepreparation of the SA device 33 for use is completed. Instead of theelectric signal, a message or data may be used to notify the preparationcompletion from the SA gas cell controller 80 to the laser controller60.

If the absolute value of the temperature deviation ΔT exceeds thepredetermined threshold Tth, on the other hand (S43: NO), the SA gascell controller 80 outputs the control signal from the gas temperaturecontroller 82 to the heat exchanger 332, and operates the heat exchanger332 so that the deviation ΔT becomes small (S45). In other words, if thetemperature of the mixed gas is higher than the reference temperatureTset, the SA gas cell controller 80 decreases the temperature of themixed gas using the heat exchanger 332.

Then the SA gas cell controller 80 outputs an NG signal to the lasercontroller 60 (S46), and processing returns to S40. The NG signal is asignal to indicate that the preparation of the SA device 33 for use isnot yet completed.

FIG. 9 is a flow chart depicting the processing for controlling gascirculation and gas temperature (S13 in FIG. 6). The SA gas cellcontroller 80 outputs an activation start signal to the circulation pump331 (1) to activate the circulation pump 331 (1) (S50). If thecirculation pump 331 (1) has already been operating, it is not necessaryto output the activation start signal.

The SA gas cell controller 80 detects the temperature TSA of the mixedgas in the SA gas cell 330 using the temperature sensor 336 (2) (S51),and calculates the deviation ΔT of the detected temperature TSA from thereference temperature Tset (S52).

The SA gas cell controller 80 determines whether the absolute value ofthe deviation ΔT is a predetermined threshold Tth or less (S53). If theabsolute value of the deviation ΔT is the predetermined threshold Tth orless (S53: YES), it is determined that the temperature of the mixed gasis maintained at the reference temperature Tset, and processing returnsto the main routine shown in FIG. 6.

If the absolute value of the deviation ΔT exceeds the predeterminedthreshold Tth (S53: NO), on the other hand, it is determined that themixed gas is not maintained at the reference temperature Tset. So the SAgas cell controller 80 outputs a control signal from the gas temperaturecontroller 82 to the heat exchanger 332 so as to decrease the deviationΔT (S54), and processing returns to the main routine.

FIG. 10 is a flow chart depicting the abnormality detection processing(S14 in FIG. 6). The SA gas cell controller 80 detects the temperatureTSA of the mixed gas in the SA gas cell 330 using the temperature sensor336 (2) (S60). Then the SA gas cell controller 80 detects the pressurePSA in the SA gas cell 330 using the pressure sensor 336 (1) (S61).

The SA gas cell controller 80 calculates the deviation ΔT of thedetected temperature TSA from the reference temperature Tset (S62), andcalculates the deviation ΔP of the detected pressure PSA from thereference pressure Pset (S63). Then it is determined whether theabsolute value of the calculated deviation ΔT is a predeterminedthreshold Tth or less (S64).

Then the SA gas cell controller 80 determines whether the absolute valueof the deviation ΔP of the pressure value PSA detected by the pressuresensor 336 (1) from the predetermined mixed gas pressure Pset is apredetermined threshold Pth or less (S65).

If the temperature and pressure of the mixed gas are both normal (S62:YES and S63: YES), then it is determined that the SA device 33 isnormally operated, and processing returns to the main routine (S66). Ifan abnormality is detected either in the temperature or pressure of themixed gas (S62: NO or S63: NO), on the other hand, it is determined thatan abnormality is generated in the SA device 33, and processing returnsto the main routine (S67).

FIG. 10 is the case when the temperature and pressure of the mixed gasare monitored, but only the temperature of the mixed gas may bemonitored instead. Or in addition to the temperature of the mixed gas,the flow rate and flow velocity of the mixed gas, that circulates in theSA device 33, may be monitored. The temperature or temperaturedistribution of the window 3332 may be monitored, as mentioned later.

FIG. 11 are diagrams depicting the principle of the wavefrontcompensator 34. The top diagram in FIG. 11 is a case when the heat loadapplied to the amplification system 30 (mainly the SA device 33) is low.The bottom diagram in FIG. 11 is the case when the heat load applied tothe amplification system 30 (mainly SA device 33) is high.

The wavefront compensator 34 has an angle compensator 100 and thewavefront curvature compensator 200. The angle compensator 100 is anoptical system for adjusting an angle (advancing direction) of the laserbeam. The wavefront curvature compensator 200 is an optical system foradjusting the curvature of the wavefront of the laser beam (e.g.divergence of the beam). A concrete structure example will be describedlater as another embodiment.

The angle compensator 100 is comprised of two reflecting mirrors, 101and 102, which are disposed to face each other in parallel. Eachreflecting mirror 101 and 102 are rotatably disposed as shown in thediagram at the bottom in FIG. 11, so that the X axis (vertical axis toFIG. 11) and the Y axis (axis perpendicular to the X axis on the sameplane) become the rotation centers respectively. In other words, eachreflecting mirror 101 and 102 are installed so as to be able to tilt androtate.

If the heat load is low, the laser beam L1 advances matching with thereference optical axis, so there is no need to change the orientation ofeach reflecting mirror 101 and 102. If the heat load is high, the laserbeam Lie enters somewhat deviated from the reference optical axis.Therefore the attitude of each reflecting mirror 101 and 102 is changedappropriately so that the output direction of the laser beam matcheswith the reference optical axis.

The wavefront curvature compensator 200 can be constituted by an convexlens 201 and a concave lens 202, for example. By adjusting the relativepositional relationship of each lens 201 and 202, the concave wave andconvex wave can be compensated to a plane wave.

The wavefront compensation controller 50, as the “compensation controlunit”, drives the angle compensator 100 and the wavefront curvaturecompensator 200 based on the measurement result by the sensor 36, so asto clear deviations from the target values. Thereby the wavefrontcompensator 34 corrects the angle and the wavefront curvature of theincoming laser beam to a predetermined angle and predeterminedcurvature, and outputs the laser beam. The wavefront compensator 34adjusts the laser beam so as to have an angle and wavefront curvature ofthe beam required for the main-amp to amplify at high efficiency, andoutputs the adjusted beam, and transforms the beam into a predeterminedlaser beam luminous flux. The transformed laser beam is amplified by themain-amp 35.

As FIG. 1 shows, the sensor 36, as the “wavefront measurement unit”, isdisposed at the downstream side of the main-amp 35, and detects theangle and wavefront curvature of the laser beam that enters from themain-amp 35. The configuration of the sensor 36 is sufficient only ifthe angle and wavefront curvature of the laser beam can be measureddirectly or indirectly.

An example of the sensor 36 will now be described with reference to FIG.12. The sensor 36 has a reflecting mirror 300 that reflects the laserbeam L1, and an optical sensor 360 for measuring a very small part ofthe laser beam L1L, that transmits through the reflecting mirror 300,for example.

The reflecting mirror 300 has a beam splitter substrate 300A coated by acoating that reflects the laser beam L1 at high reflectance, and aholder with a water-cooling jacket 300B for holding the beam splittersubstrate 300A.

The beam splitter substrate 300A is constituted by such a substance assilicon (Si), zinc selenide (ZnSe), gallium arsenide (GaAs) and diamond.Most of the laser beam L1 is reflected by the high reflecting coating ofthe beam splitter substrate 300A, but a very small part of the laserbeam L1L transmits through the beam splitter substrate 300A.

The very small part of the laser beam L1L that transmitted through thebeam splitter substrate 300A enters the optical sensor unit 360 as asample light. For the optical sensor unit 360, a beam profiler formeasuring the intensity distribution of the laser beam, power sensor(calorimeter, pyro sensor) for measuring the laser duty and load of anoptical element, and a wavefront sensor that can simultaneously measurethe wavefront state of the laser beam and beam traveling direction(angle), for example, can be used.

As mentioned later, the wavefront state and angle (direction) of thelaser beam may be measured using parameters related to the state of thelaser beam (e.g. temperature of SA gas cell 330) and a data baseacquired from simulation and experiment results.

FIG. 13 is a flow chart depicting the wavefront compensation processingthat is executed by the wavefront compensation controller (WFC-C) 50.This processing is executed at startup before the laser light sourcedevice 2 starts operation. In other words, in the adjustment stage ofthe laser light source device 2 before starting operation, a shutter,which is not illustrated, is closed before the laser beam is irradiatedonto the target, for example, so that the laser beam does not enter theEUV chamber 10, and then the laser is oscillated and adjusted. When theseed light is output from the laser oscillator 20, the wavefront andangle (direction) of the laser beam line downstream from the laseroscillator 20 is adjusted so that the amplification efficiency of themain-amp 35 can be maintained as high.

The wavefront compensation controller 50 acquires the measured valuefrom the sensor 36 (S100), and calculates the deviation ΔD, which is adifference between the target value and the measured value (S101). Thewavefront compensation controller 50 determines whether the absolutevalue of ΔD is a predetermined allowed value Dth or less (S102). Theallowable value Dth is set as a value which does not negativelyinfluence the amplification characteristic of the laser beam, forexample.

If the difference ΔD between the target value and the measured value isthe allowed value Dth or less (S102: YES), the wavefront correctioncontroller 50 outputs the OK signal to the laser controller 60 (S103).The OK signal is an adjustment completion signal to indicate that thewavefront of the laser beam was adjusted to a predetermined wavefront(curvature and direction).

If the absolute value of ΔD exceeds the allowed value Dth (S102: NO), onthe other hand, the wavefront compensation controller 50 outputs an NGsignal to the laser controller 60 (S104). The NG signal is an adjustmentincompletion signal to indicate that the wavefront of the laser beam hasnot yet been adjusted to the predetermined wavefront.

The wavefront compensation controller (WFC-C) 50 outputs the drivesignal to the wavefront compensator (WFC) 34, and has the wavefrontcompensator (WFC) 34 execute the compensation operation (S105). Thewavefront compensator (WFC) 34 operates the angle compensator 100 andthe wavefront curve compensator 200 according to the drive signal. Byexecuting the compensation operation once or a plurality of times, thewavefront of the laser beam is matched with the predetermined wavefront.

FIG. 14 is a flow chart depicting the operation of the laser controller60 and the operation of the EUV light source controller 70. When thelaser controller 60 receives an OK signal from the wavefrontcompensation controller (WFC-C) 50 (S110: YES), the laser controller 60notifies the EUV light source controller 70 that adjustment of the laserlight source device 2 has completed (5111).

The EUV light source controller 70 determines whether the OK signal wasreceived from the laser controller (S112). When the adjustmentcompletion notice or the OK signal is received from the laser controller60, the EUV light source controller 70 supplies the droplet DP from thetarget material supply unit 15 to the chamber main body 11 (S113). Thenthe laser beam is irradiated onto the target (S114). Then it isdetermined whether the OK signal was received, and if the OK signal wasnot received, the EUV light source controller 70 cancels supplying thetarget and irradiating the laser beam onto the target. If the OK signalwas received from the laser controller, on the other hand, the EUV lightsource controller 70 supplies the target (S113) and irradiates the laserbeam onto the target (S114).

The laser controller 60 outputs the laser beam L1 from the laseroscillator 20 at a timing matching with the droplet DP supply timing.The laser beam L1 is amplified by the amplification system 30, and thenenters the chamber 10 via the focusing system 40. The droplet DP, whichis irradiated with the laser beam L1, turns into plasma PLZ. The EUVlight L2 radiated from the plasma PLZ is collected to the intermediatefocus IF by the EUV collector mirror 14, and is sent to the EUV exposuredevice 5.

The present embodiment having the above configuration exhibits thefollowing effects. According to the present embodiment, the mixed gasincluding the SA gas is circulated and used in the SA device 33, and thetemperature of the mixed gas is maintained to be a predetermined value,so the case of the EUV light source device 1, being operatedcontinuously for a long time, can be supported. In other words, the EUVlight source device 1 can operate the SA device 33 normally even when ahigh load state continues for a long time, and can supply a laser beamhaving stable pulse energy and pulse waveforms to the EUV light sourcedevice 1.

According to the present embodiment, each inlet 3336 (1) and 3336 (2)are disposed near the inner face side of the input window 3332 (1), soas to be axially symmetric with respect to the optical axis of the laserbeam, and each outlet 3338 (1) and 3338 (2) are disposed near the innerface side of the output window 3332 (2), so as to be axially symmetricwith respect to the optical axis of the laser beam. Therefore thetemperature distribution of the windows 3332 (1) and 3332 (2) can beaxially symmetric with respect to the optical axis of the laser beam bythe mixed gas that flows to the inner face side of the windows 3332 (1)and 3332 (2). Hence the wavefront of the laser beam that transmittedthrough the SA gas cell 330 can be an axially symmetric wavefront, andthe change of the wavefront can be minimized. As a result, the wavefrontcompensation by the wavefront compensator 34 can be easily executed.

According to the present embodiment, the wavefront compensator 34 foradjusting the curvature and direction of the laser beam, and the sensor36 for detecting the curvature and direction of the wavefront of thelaser beam, are disposed. Therefore according to the present embodiment,the curvature and direction (angle) of the wavefront of the laser beamthat transmits through the SA gas cell 330 can be adjusted beforestarting operation of the laser light source device 2 by the wavefrontcompensator 34. Therefore even if a heat load is high in the operationstate, the output characteristic of the laser beam can be stabilized.

Embodiment 2

A second embodiment will now be described with reference to FIG. 15.Each of the following embodiments is a variant form of the firstembodiment. Therefore the main differences from the first embodiment aredescribed. In the present embodiment, a total of four inlets 3336 (1),3336 (2), 3336 (3) and 3336 (4) are disposed so that the SA gas turnsaround the inner surface of each window 3332 (1) and 3332 (2). In thepresent embodiment, two outlets 3338 (1) and 3338 (2) are disposedbetween each window 3332 (1) and 3332 (2).

FIG. 15 is a diagram depicting an SA gas cell 330A. The drawing at theleft in FIG. 15 shows a view from the input window 3332 (1), and thedrawing at the center in FIG. 15 shows a side view of the SA gas cell330A, and the drawing at the right in FIG. 15 shows a view from theoutput window 3332 (2).

Each inlet 3336 (1) and 3336 (2) at the input window side 3332 (1) istilted at angle θ1, so that the SA gas turns and flows around the innersurface of the input window 3332 (1), and the inlets are disposed inparallel with the tangential line of the input window 3332 (1). Eachinlet 3336 (1) and 3336 (2) is disposed so as to be axially symmetricwith respect to the optical axis of the laser beam L1. The mixed gasthat flows in from each inlet 3336 (1) and 3336 (2) flows to the centerof the input window 3332 (1) while turning around in a predetermineddirection, as shown at the left side of FIG. 15.

Then the mixed gas flows in a flow space 3334 along the optical axis ofthe laser beam approximately to the center of the flow space 3334. Thetwo outlets 3338 (1) and 3338 (2) are disposed along the diameterdirection so as to be connected to the center portion of the flow space3334. Each outlet 3338 (1) and 3338 (2) is axially symmetric withrespect to the optical axis of the laser beam.

While moving approximately to the center of the flow space 3334, themixed gas absorbs the feeble light, not greater than a predeterminedvalue, included in the laser beam L1, and increases its temperature. Themixed gas flows out from each outlet 3338 (1) and 3338 (2), and is sentto the heat exchanger 332 via the exhaust pipeline 333 (2).

As described on each inlet 3336 (1) and 3336 (2) at the input window3332 (1) side, each inlet 3336 (3) and 3336 (4) at the output window3332 (2) side is tilted at angle θ1, so that the SA gas turns and flowsaround the inner surface of the output window 3332 (2), and the inletsare disposed in parallel with the tangential line of the output window3332 (2). Each inlet 3336 (3) and 3336 (4) is disposed so as to beaxially symmetric with respect to the optical axis of the laser beam L1.The mixed gas that flows in from each inlet 3336 (3) and 3336 (4) flowsto the center of the output window 3332 (2) while turning around in apredetermined direction, as shown at the right side of FIG. 15.

Then the mixed gas that turned around the inner surface of the outputwindow 3332 (2) moves in the flow space 3334 along the optical axis ofthe laser beam toward the outlets 3338 (1) and 3338 (2). While moving,the mixed gas absorbs feeble light, not greater than a predeterminedvalue, included in the laser beam.

The turning direction of the mixed gas at the input window 3332 (1) sideand the turning direction of the mixed gas at the output window 3332 (2)side are the same.

The present embodiment having this configuration also has effectssimilar to the first embodiment. Furthermore according to the presentembodiment, a plurality of inlets, in parallel with the tangentialdirection of each window, are disposed on both the input window 3332 (1)and the output window 3332 (2), so that the mixed gas turns around andflows on the inner surface of each window. Since the mixed gas passes onthe inner surface of each window 3332 (1) and 3332 (2) while turningaround each window, the temperature distribution of each windows 3332(1) and 3332 (2) can be axially symmetric with respect to the opticalaxis of the laser beam.

Embodiment 3

A third embodiment will now be described with reference to FIG. 16 andFIG. 17. In the present embodiment, porous cylindrical tubes 3340 (1)and 3340 (2), as “flow control elements”, are disposed in the windows3332 (1) and 3332 (2) respectively, so that the mixed gas in the flowspace 3334 uniformly moves to each outlet 3338 (1) to 3338 (4). FIG. 16is a diagram depicting an SA gas cell 330B according to the presentembodiment. FIG. 17 is a cross-sectional view of a porous cylindricaltube 3340.

As the drawing at the left in FIG. 16 shows, two outlets 3338 (1) and3338 (2) are disposed in the diameter direction so as to be axiallysymmetric with respect to the optical axis of the laser beam. The porouscylindrical tube 3340 (1) is disposed between the outer circumferenceside of the inner face of the input window 3332 (1) and the outlets 3338(1) and 3338 (2).

As the drawing at the right in FIG. 16 shows, two outlets 3338 (3) and3338 (4) are also disposed in the diameter direction, so as to beaxially symmetric with respect to the optical axis of the laser beam.The porous cylindrical tube 3340 (2) is also disposed between the outercircumference side of the inner face of the output window 3332 (2) andthe outlets 3338 (3) and 3338 (4).

Between each window 3332 (1) and 3332 (2), two inlets 3336 (1) and 3336(2) are disposed in the diameter direction, so as to be axiallysymmetric with respect to the optical axis of the laser beam. The mixedgas flows in from each inlet 3336 (1) and 3336 (2) to the center portionof the flow space 3334. The mixed gas moves along the optical axis ofthe laser beam toward each outlet 3338 (1) to 3338 (4) at the left andright, while absorbing feeble light included in the laser beam. Themixed gas flows into each outlet 3338 (1) to 3338 (4) via the porouscylindrical tubes 3340 (1) and 3340 (2), and is exhausted to the exhaustpipeline 333 (2) via each outlet 3338 (1) to 3338 (4).

Now the cross-sectional view in FIG. 17 is referred to. FIG. 17 is across-sectional view depicting one side of the porous cylindrical tube3340 (1). Description of the other side of the porous cylindrical tube3340 (2), which is constructed in the same way as the one side of theporous cylindrical tube 3340 (1), is omitted.

The porous cylindrical tube 3340 (1) is coaxially disposed with thewindows 3332 (1) and 3332 (2). As a result, the center of the porouscylindrical tube 3340 (1) approximately matches the center of theoptical axis of the laser beam.

The porous cylindrical rube 3340 (1) has a collar portion 3341 andtubular portion 3342 which is integrated with the collar portion 3341.Many small holes 3343 for letting mixed gas flow are formed on thetubular portion 3342. The small holes 3343 correspond to the “flowholes”. The tubular portion 3342 is formed to have a tubular shape,constituted by porous alumina ceramic or porous stainless sinteredsteel, for example.

One edge (left hand side in FIG. 17) of the tubular portion 3342contacts the inner wall of the holder 3300, which is disposed at theinner face side of the input window 3332 (1). The other edge of thetubular portion 3342 (right hand side in FIG. 17) is integrated with thering-shaped collar portion 3341.

The inner circumference side of the collar portion 3341 is integratedwith the other edge of the tubular portion 3342, and the outercircumference side of the collar portion 3341 contacts the innercircumference of the holder 3300. The small holes are not formed in thecollar portion 3341, therefore mixed gas cannot pass through the collarportion 3341 in any substantial way. However the collar portion 3341 maybe constituted by a porous material, so that mixed gas can flow through.

In the present embodiment, each outlet 3338 (1) to 3338 (4) are disposedat each window 3332 (1) and 3332 (2) side, and the porous cylindricaltubes 3340 (1) and 3340 (2), that have many small holes 3343, aredisposed so as to cover each outlet 3338 (1) to 3338 (4). The mixed gasthat flows into the tubular portions 3342 of the porous cylindricaltubes 3340 (1) and 3340 (2) flow to the outlet side via each small hole3343. As the drawings at the left and right of FIG. 16 show, the mixedgas flows out radially from the center axis of the SA gas cell 330B(this center axis approximately matches the optical axis of the laserbeam).

Therefore according to the present embodiment, the mixed gas canradially and uniformly flow around the optical axis of the laser beam atthe inner face side of the windows 3332 (1) and 3332 (2). Thereby thetemperature distribution of the mixed gas that flows at the inner faceside of each window 3332 (1) and 3332 (2) can be made axially symmetricwith respect to the laser optical axis. As a result, the temperaturedistribution of each window 3332 (1) and 3332 (2) can be axiallysymmetric with respect to the optical axis of the laser beam.

Hence according to the present embodiment, just like the aboveembodiments, the wavefront profile of the laser beam that transmitsthrough the SA gas cell 330B can be axially symmetric, and compensationby the wavefront compensator 34 can be executed easily.

Embodiment 4

The fourth embodiment will now be described with reference to FIG. 18.According to the present embodiment, a plurality of SA gas cells 330C(1) to 330C (4) are linked, and the flow directions of the mixed gas inadjacent SA gas cells are set to be opposite from each other. Alsoaccording to the present embodiment, the windows 3332 (2) to 3332 (4),between each SA gas cell 330C (1) to 330C (4), are commonly used.

The block diagram in FIG. 18 shows the SA device 33C in a state wherethe SA gas cell controller 80, gas temperature controller 82, lasercontroller 60, pressure sensor 336 (1) and temperature sensor 336 (2)are removed for convenience of description.

A total of four SA gas cells 330C (1) to 330C (4) are disposed next toeach other, and are parallel with the supply pipeline 333 (1) andexhaust pipeline 333 (2). Two adjacent SA gas cells form a pair. Thefirst SA gas cell 330C (1) and the second SA gas cell 330C (2) form afirst pair. The third SA gas cell 330C (3) and the fourth SA gas cell330C (4) form a second pair. The flow directions of the mixed gas areopposite in the adjacent SA gas cells of each pair.

In the example in FIG. 18, the mixed gas in the first SA gas cell 330C(1) flows from bottom to top in FIG. 18, and the mixed gas in the secondSA gas cell 330C (2), adjacent to the first SA gas cell 330C (1), flowsfrom the top to bottom in FIG. 18. The mixed gas in the third SA gascell 330C (3), adjacent to the second SA gas cell 330C (2), flows frombottom to top in FIG. 18, and the mixed gas in the fourth SA gas cell330C (4), adjacent to the third SA gas cell 330C (3), flows from top tobottom.

According to the present embodiment, where each SA gas cell 330C (1) to330C (4) are linked, the windows between the SA gas cells can be shared.The first window 3332 (1) is an input window of the first SA gas cell330C (1). The second window 3332 (2) is an output window of the first SAgas cell 330C (1), and an input window of the second SA gas cell 330C(2). The third window 3332 (3) is an output window of the second SA gascell 330C (2), and an input window of the fourth SA gas cell 330C (4).The fifth window 3332 (5) is an output window of the fourth SA gas cell330C (4).

According to the present embodiment, flow directions of the mixed gas inadjacent SA gas cells are opposite from each other. When the laser beamtransmits through one of SA gas cells, the wavefront thereof isdistorted in one direction according to the temperature distribution ofthe window. In the other of SA gas cells, the flow direction of themixed gas is the opposite of the flow direction of the one of SA gascells, and similarly its temperature distribution is opposite thetemperature distribution of the one of SA gas cells. Therefore when themixed gas transmitted through the one of SA gas cells transmits throughthe other of SA gas cells, a function to distort the wavefront in theopposite direction is generated, and the generation of temperaturedistribution in the intermediate portion windows 3332 (2), 3332 (3) and3332 (4) is suppressed, and as a result, a change of wavefront of thelaser beam is suppressed.

Therefore the present embodiment has the same effects as the abovementioned embodiments. According to the present embodiment, the case ofdisposing the gas pipelines of each SA gas cell 330C (1) to 330C (4) inparallel was described, but the gas pipelines of each SA gas cell 330C(1) to 330C (4) may be disposed in series instead.

In other words, the supply pipelines 333 (1) is connected only to theinlet of the first SA gas cell 330C (1), the outlet of the first SA gascell 330C (1) is connected to the inlet of the second SA gas cell 330C(2), and the outlet of the second SA gas cell 330C (2) is connected tothe inlet of the third SA gas cell 330C (3). The outlet of the third SAgas cell 330C (3) may be connected to the inlet of the fourth SA gascell 330C (4), and the exhaust pipeline 333 (2) may be connected to theoutlet of the fourth SA gas cell 330C (4).

Also according to the present embodiment, a case of letting the mixedgas flow from top to bottom, or from bottom to top in FIG. 18, wasdescribed. However the mixed gas may flow in another direction, such asthe lateral direction or diagonal direction, when a window is viewedfrom the front face instead. It is sufficient only if the flowdirections of the mixed gas in adjacent SA gas cells are set to beopposite from each other.

Also according to the present embodiment, a case of linking four SA gascells 330C (1) to 330C (4) was described, but two or six or greater evennumber of SA gas cells may be linked instead. Thereby the windows can beshared, and the manufacturing cost of the SA devices can be decreased.

Also according to the present embodiment, a plurality of SA gas cells330C (1) to 330C (4) are linked sharing windows, but the presentinvention is not limited to this, for each SA gas cell may be disposedindependently such that the flow directions of the mixed gas in theadjacent SA gas cells can be opposite from each other.

Embodiment 5

A fifth embodiment will now be described with reference to FIG. 19 andFIG. 20. In the present embodiment, variant forms of the positionalrelationship of the wavefront compensator 34, sensor 36 and SA device 33will be described. Normally the laser beam transmits through the SA gascell 330, but in this embodiment, it is assumed that the laser beamtransmits through the SA device 33 for convenience of description.

FIG. 19A shows a configuration where the wavefront compensator 34 andthe sensor 36 are disposed in the downstream side of the SA device 33.The laser beam, as a plane wave, transmits through the SA device 33 andthe wavefront thereof becomes a convex wavefront. The direction of thewavefront of the laser beam that transmits through the SA device 33 isinclined downward in FIG. 19A. The wavefront of the laser beam that istransmitted through the SA device 33 is compensated by the wavefrontcompensator 34, and the compensated laser beam is input to the sensor36.

The wavefront compensation controller (WFC-C) 50 controls the wavefrontcompensator 34 based on the beam characteristics detected by the sensor36, so as to maintain predetermined beam characteristics. In thisembodiment, the laser beam is output as a plane wave. The predeterminedbeam characteristics refer to a plane wave of which traveling direction(angle) is the same as the direction before the laser beam entered theSA device 33.

FIG. 19B shows a case when the wavefront compensator 34 is disposed atthe upstream side of the SA device 33, and the sensor 36 is disposed atthe downstream side of the SA device 33. The wavefront of the laser beamis compensated by the laser beam passing through the wavefrontcompensator 34. The laser beam of which wavefront was compensatedtransmits through the SA device 33, and then enters the sensor 36. Basedon the beam characteristics detected by the sensor 36, the wavefrontcompensation controller 50 controls the wavefront compensator 34, so asto maintain predetermined beam characteristics.

FIG. 19C shows a case when the wavefront compensator 34 and the sensor36 are disposed in this sequence at the upstream side of the SA device33. The laser beam transmits through the wavefront compensator 34 andreaches the sensor 36. Based on the beam characteristics detected by thesensor 36, the wavefront compensation controller 50 controls thewavefront compensator 34 to maintain predetermined beam characteristics.The laser beam after passing through the sensor 36 enters the SA device33. The laser beam is transformed into a plane wave by transmittingthrough the SA device 33, and is output. In this example, the wavefrontcompensation controller 50 estimates the distortion of the wavefront inthe SA device 33, and controls the wavefront compensator 34 so that thewavefront returns to normal when the laser beam transmits through thewavefront compensator 34 and the SA device 33.

As shown in FIG. 20, a plurality of sensors 36 or a plurality ofwavefront compensators 34 may be disposed. In FIG. 20A, the wavefrontcompensator 34 and the sensor 36 (1) are disposed in this sequence atthe upstream side of the SA device 33, and another sensor 36 (2) isdisposed at the downstream side of the SA device 33.

The laser beam transmits through the wavefront compensator 34, and isinput to the first sensor 36 (1). The first sensor 36 (1) measures thecharacteristics of the laser beam that is input to the SA device 33, andsends the measurement result to the wavefront compensation controller50.

After transmitting through the SA device 33, the laser beam is input tothe second sensor 36 (2). The second sensor 36 (2) measures thecharacteristics of the laser beam, and sends it to the wavefrontcompensation controller (WFC-C) 50. Based on the two beamcharacteristics detected by the first sensor 36 (1) and the secondsensor 36 (2), the wavefront compensation controller (WFC-C) 50 controlsthe wavefront compensator (WFC) 34, so as to reach predetermined beamcharacteristics respectively at the positions of both sensors. Afterpassing through the second sensor 36 (2), the laser beam is output as aplane wave.

FIG. 20B shows a case when the first wavefront compensator (WFC) 34 (1)and the first sensor 36(1) are disposed at the upstream side of the SAdevice 33, and the second wavefront compensator (WFC) 34 (2) and thesecond sensor 36(2) are disposed at the downstream side of the SA device33.

After transmitting through the wavefront compensator (WFC) 34(1), thelaser beam is input to the sensor 36 (1). The sensor 36 (1) measures thecharacteristics of the laser beam that were input to the SA device 33,and outputs the measurement result to the wavefront compensationcontroller (WFC-C) 50. The laser beam that passed through the sensor 36(1) transmits through the SA device 33, and is input to the wavefrontcompensator (WFC) 34 (2). After transmitting through the wavefrontcompensator (WFC) 34 (2), the laser beam is input to the sensor 36 (2).The sensor 36 (2) detects the characteristics of the laser beam, andoutputs it to the wavefront correction controller (WFC-C) 50. Based onthe two beam characteristics detected by the first sensor 36 (1) and thesecond sensor 36 (2), the wavefront compensation controller (WFC-1) 50controls the first wavefront compensator (WFC) 34 (1) and the secondwavefront compensator (WFC) 34 (2), so that predetermined beamcharacteristics are implemented respectively at the positions of thesesensors.

Embodiment 6

The sixth embodiment will now be described with reference to FIG. 21. Inthe present embodiment, a case of constituting the wavefront curvaturecompensator 200 by a transmission optical system will be described. AsFIG. 21 shows, the wavefront curvature compensator 200 can beconstituted by a convex lens 201 and a concave lens 202.

FIG. 21A shows a state of outputting a plane wave, which was input, as aplane wave. If the focal position of the convex lens 201 and the focalposition of the concave lens 202 match at a confocal point cf, then thelaser beam in the plane wave state that transmitted through the convexlens 201 is transformed into a concave wavefront. The laser beam of theconcave wavefront that passed through the concave lens 202 istransformed into a plane wave.

FIG. 21B shows a state of transforming a convex wave into a plane wave.The convex lens 201 is shifted to the upstream side (left side in FIG.21) from the position shown in FIG. 21A. The focal position F1 of theconvex lens 201 and the focal position F2 of the concave lens are bothon the optical axis of the laser beam, and the focal point F1 of theconvex lens 201 is more to the upstream side.

When a laser beam is changed into a convex wave due to the influence ofheat in the SA device 33, the laser beam enters the convex lens 201 in astate of divergent light, and is changed into a concave wave by theconvex lens 201. The laser beam transformed into a concave wavetransmits through the concave lens 202, and thereby is transformed intoa plane wave.

FIG. 21C shows a state of transforming the concave wave into a planewave. The focal position F1 of the convex lens 201 and the focalposition F2 of the concave lens 202 are on a same optical axis, and thefocal position F2 is disposed at the upstream side of the focal positionF1. When the laser beam of the convex wave enters the convex lens 201,the convex wave is transformed into a concave wave. The laser beam ofthe concave wave passes through the concave lens 202, and thereby istransformed into a plane wave.

FIG. 21D shows an example of constructing the wavefront curvaturecompensator 200 using two convex lenses 201 and 203. The convex lens 201can shift in the lateral direction (optical axis direction) in FIG. 21,by a single axis stage 204.

To output a laser beam, that is input as a plane wave (parallel light),as a plane wave (parallel light), the position of the convex lens 201 isset so that the focal position of the convex lens 201 and the focalposition of the convex lens 203 match.

If the laser beam becomes convergent light (concave wavefront) by a heatload, the convex lens 201 is shifted to the downstream position 201R onthe optical axis by the single axis stage 204. If the laser beam becomesdivergent light (convex wavefront), on the other hand, the convex lens201 is shifted to the upstream position 201L on the optical axis by thesingle axis stage 204.

Embodiment 7

A seventh embodiment will now be described with reference to FIG. 22.According to the present embodiment, an example of a case ofconstructing the wavefront curvature compensator 200B as a reflectionoptical system will be described. The wavefront curvature compensator200B has two reflecting mirrors 205 (1) and 205 (2), and two off-axisparabolic concave mirrors 206 (1) and 206 (2). The reflecting mirror 205(1) and the off-axis parabolic concave mirror 206 (1), which arepositioned at the upper side in FIG. 22, are installed in plate 207. Theplate 207 is moveable in the vertical direction in FIG. 22. Each mirror205 (1) and 206 (1) moves vertically along with the plate 207.

FIG. 22A is an arrangement in the case when the laser beam that wasinput as parallel light (plane wave) is output still as parallel light(plane wave). In this case, the focal position of the off-axis parabolicconcave mirror 206 (1) and the focal position of the off-axis parabolicconcave mirror 206 (2) are matched to be a state of the confocal pointcf.

The laser beam enters from the left side (upstream) in FIG. 22 to thereflecting mirror 205 (2), is reflected, and enters another reflectingmirror 205 (1). The laser beam reflected by this reflecting mirror 205(1) enters the off-axis parabolic concave mirror 206 (1). The laser beamis reflected by the off-axis parabolic concave mirror 206 (1) at a 45degrees reflection angle, and is focused at the focal position cf. Thelaser beam diverged from the focal position cf and enters the off-axisparabolic concave mirror 206 (2), and is reflected at a 45 degreesreflection angle.

FIG. 22B is an arrangement when the laser beam that is input asconvergent light (concave wavefront) is transformed into parallel light(plane wave), and is output. In this case, the plate 207 is shifteddownward, and the focal point position f of the off-axis parabolicconcave mirror 206 (1) is shifted to downstream on the optical axis.Thereby the focal point position of the off-axis parabolic concavemirror 206 (1) and the focal position of the off-axis parabolic concavemirror 206 (2) are matched on the optical axis.

To enter the laser beam as divergent light (convex wavefront), the plate207 is shifted to the upper side in FIG. 22.

In the wavefront curvature compensator 200B that has this configuration,the reflecting mirror 205 (1) and the off-axis parabolic concave mirror206 (1) are secured on the plate 207, and both the mirrors 205 (1) and206 (1) are simultaneously shifted on the optical axis (verticaldirection in FIG. 22). Thereby according to the present embodiment, theoptical axis of the input light and the optical axis of the output lightare matched, and the wavefront curvature can be compensated.

The wavefront curvature compensator 200B of the present embodiment isconstructed as a reflection optical system, so even if the laser beampasses through the wavefront curvature compensator 200B, the change ofthe wavefront due to heat of the wavefront curvature compensator itselfcan be minimized. Therefore even if a high output laser beam is used,the wavefront curvature can be compensated at high precision.

Embodiment 8

An eighth embodiment will now be described with reference to FIG. 23.The wavefront curvature compensator 200C of the present embodiment isconstituted by a reflection optical system including an off-axisparabolic concave mirror 206, an off-axis parabolic convex mirror 208,and two reflecting mirrors 205(1) and 205(2).

The off-axis parabolic concave mirror 206 and the reflection mirror 205(1) are installed on the plate 207 that can move vertically. The focalposition of the off-axis parabolic convex mirror 208 and the focalposition of the off-axis parabolic concave mirror 206 are disposed so asto match at cf.

The laser beam of the plane wavefront is reflected by the off-axisparabolic convex mirror 208, enters the off-axis parabolic concavemirror 206 as divergent light, and is transformed into a plane wave. Thelaser beams of the plane wave are reflected by the reflecting mirrors205 (1) and 205 (2), and are output. Just like the seventh embodiment,the wavefront of the input laser beam can be corrected to the plane waveand output by shifting the plate 207 vertically.

The present embodiment constructed like this also exhibits the effects,just like the seventh embodiment. Furthermore according to the presentembodiment, the distance between both the off-axis parabolic mirrors canbe decreased by combining the off-axis parabolic concave surface 206 andthe off-axis parabolic convex surface 208. As a result, generaldimensions can be decreased compared with the seventh embodiment.

Embodiment 9

A ninth embodiment will now be described with reference to FIG. 24 andFIG. 25. According to the present embodiment, the wavefront curvaturecompensators 200D and 200E are constructed as one convex mirror 209 andone concave mirror 210 which are disposed in a Z shape.

FIG. 24 shows a wavefront curvature compensator 200D constituted by thespherical convex mirror 209 at the upstream side and the sphericalconcave mirror 210 at the downstream side in a Z shape. For example, ifa laser beam of divergent light (convex wavefront) enters the convexmirror 209, the convex mirror 209 reflects the laser beam at an incidentangle α that is 3 degrees or less. The reflected laser beam enters theconcave mirror 210 at the incident angle α, and is transformed intoparallel light (plane wave).

For example, as the arrow mark in FIG. 24 shows, the wavefront of thelaser beam can be transformed into a plane wave by shifting the positionof the concave mirror 210 along the reflection optical axis of theconvex mirror 209.

FIG. 25 shows a wavefront curvature compensator 200E constructed by theconcave mirror 210 at the upstream side and the convex mirror 209 at thedownstream side which are disposed in a Z shape. For example, if thelaser beam of divergent light (convex wavefront) enters the concavemirror 210, the concave mirror 210 reflects the laser beam at smallincident angle α (e.g. 3 degrees or less). The reflected laser beamenters the convex mirror 209 at the incident angle α, and is transformedinto parallel light (plane wave). For example, the curvature ofwavefront of the laser beam can be transformed into a plane by shiftingthe position of the convex mirror 209 along the reflection light axis ofthe concave mirror 210.

In this way, according to the present embodiment, the wavefrontcurvature compensator can be constituted by the convex mirror 209 andthe concave mirror 210, so the manufacturing cost can be decreased.Since a reflection optical system is used, the change of wavefrontgenerated when the laser beam passes through the wavefront curvaturecompensator can be minimized.

According to the present embodiment, the optical axis of the outputlaser beam parallel-shifts from the optical axis of the input laserbeam. Hence an optical system for matching the optical axis of theoutput light with the optical axis of the input light may be added tothe present embodiment.

Embodiment 10

The tenth embodiment will now be described with reference to FIG. 26.According to the present embodiment, a variable mirror that can variablycontrol the curvature of the reflecting plane by a control signal fromthe wavefront compensation controller (WFC-C) 50 is used. Such avariable mirror is called a VRWM (Variable Radius Wavefront Mirror) inthe present embodiment.

A wavefront curvature compensator 200H of the present embodiment isconstituted by a VRWM which has a 45 degrees incident angle. FIG. 26Ashows a case when the laser beam that enters as a plane wave (parallellight) is output as a plane wave (parallel light). When a plane wave isoutput as a plane wave, the surface of the VRWM is controlled to beflat.

FIG. 26B shows a case when the laser beam with a convex surface(divergent light) is transformed into a laser beam of a plane wave(parallel light). In this case, the shape of the VRWM is controlled tobe a toroidal-shaped convex surface.

FIG. 26C shows a case when the laser beam with a concave surface(convergent light) is transformed into a laser beam of a plane wave(parallel light). In this case, the shape of the VRWM is controlled tobe a toroidal-shaped convex surface.

According to the present embodiment having this configuration, thewavefront curvature compensator 200H can be constructed only by VRWM, socompactness can be achieved with fewer components, and efficiency isalso high, since compensation is completed by reflection performed onlyonce. Furthermore, the wavefront curvature compensator 200H of thepresent embodiment can shift the optical axis of the incident laser beamat 45 degrees, and output it. Therefore the wavefront curvaturecompensator 200H of the present embodiment can be used instead of areflecting mirror for shifting the optical path of the laser beam by 45degrees. A VRM with a 45 degrees incident angle is an optical element bywhich the mirror surface changes into a toroidal shape, and the planewave is reflected with a 45 degrees incident angle, so that thewavefront can be transformed into a wavefront of a spherical surfacewith the desired radius of curvature.

Embodiment 11

An eleventh embodiment will now be described with reference to FIG. 27.According to the present embodiment, a VRWM 213 that can change thesurface of a mirror into a spherical shape with the desired radius ofcurvature and a reflecting mirror 214 are disposed in a Z shape, wherebya wavefront curvature compensator 200J is constructed.

As FIG. 27A shows, if the laser beam that enters the VRWM as a planewave is output as a plane wave, the VRWM 213 is controlled to be a flatshape. As FIG. 27B shows, if the laser beam that enters as a convex waveis transformed into a plane wave, the shape of the VRWM 213 is set to bea concave spherical shape. As FIG. 27C shows, if the laser beam thatenters as a concave wave is transformed into a plane wave, the shape ofthe VRWM is set to be a convex spherical shape.

The present embodiment having this configuration also exhibits similareffects as the tenth embodiment. According to the present embodiment,however, the incoming optical axis and the outgoing optical axis of thelaser beam are parallel-shifted, and do not match. Hence an opticalsystem to return the optical axis to its original state may be added tothe present embodiment.

Embodiment 12

A twelfth embodiment will now be described with reference to FIG. 28. Inthe present embodiment, a wavefront compensator 34A, which can functionas both an angle compensator and a wavefront curvature compensator, willbe described. The wavefront compensator 34A has an VRWM 110 and areflecting mirror 111.

FIG. 28A shows a case when the heat load is low. The laser beam of theplane wave enters the reflecting mirror 111 at 45 degrees and isreflected, and enters the VRWM 110 at a 45 degrees incident angle. TheVRWM 110 is controlled to be a flat shape. The laser beam is reflectedby a flat mirror surface of the VRWM 110, and is output in a state of aplane wave.

The present embodiment is not limited to the case of transforming theincoming light of the plane wave into an outgoing light of the planewave. The focal distance of the VRWM can be controlled to be a constantvalue so that the laser beam that is input as divergent light (convexwavefront) is output as a laser beam having a wavefront with a desiredcurvature.

FIG. 28B shows a case when the angle (direction) and wavefront curvatureof the laser beam changed. It is assumed that the direction of theincoming laser beam inclines downward in FIG. 28, and the wavefrontthereof changed to be divergent light (convex wavefront) due to theinfluence of a heat load. In this case, the angle of the reflectingmirror 111 is controlled so that the optical axis of the laser beamreflected by the reflecting mirror 111 matches with the referenceoptical axis.

The laser beam reflected by the reflecting mirror 111 enters the VRWM110 at a 45 degrees incident angle. The shape of the VRWM 110 is set tobe a toroidal concave shape, so that the laser beam reflected by theVRWM 110 becomes a plane wave.

The above is the case of transforming the laser beam of a convex waveinto a plane wave, but the present invention is not limited to this. Thelaser beam of a concave wave could be transformed into a plane wave, orthe incoming light of a convex wavefront or a concave wavefront could betransformed into an outgoing light having a wavefront with a desiredcurvature.

In the case of an incident angle which is within an allowableaberration, the optical axis of the outgoing light may be matched withthe reference optical axis by controlling the angle of two axes, thehorizontal direction and vertical direction, of the VRWM 110 (bycontrolling tilt and roll).

Embodiment 13

A thirteenth embodiment will now be described with reference to FIG. 29.In the present embodiment, a reflecting mirror 113 and a VRWM 112 aredisposed in a Z shape, whereby a wavefront compensator 34B, which canfunction as both an angle compensator and a wavefront curvaturecompensator, is constructed. The incident angle is 2.5 degrees.

FIG. 29A shows a case when the heat load is low. The laser beam of theplane wave enters the reflecting mirror 113 at a 2.5 degrees incidentangle and is reflected. The reflected laser beam enters the VRWM 112 asa 2.5 degrees angle. The shape of the VRWM 112 is controlled to be flat,so as to reflect the laser beam in a plane wave state. The case of aplane wave was described, but the present invention is not limited tothis, but even when the convex wave or concave wave is input, forexample, a laser beam having a wavefront with a predetermined curvaturecan be output by changing the shape of the VRWM 112.

FIG. 29B shows a case when the heat load is high. The case when theangle of the incoming laser beam inclines downward in FIG. 29, and thewavefront of the laser beam becomes a convex wave will be described. Inthis case, the angle of the reflecting mirror 113 is changed so that theoptical axis of the laser beam that is reflected by the reflectingmirror 113 matches with the reference optical axis (optical axis shownin FIG. 29A).

The light reflected by the reflecting mirror 113 enters the VRWM 112 ata 2.5 degrees incident angle. The shape of the VRWM 112 is changed to bea spherical convex shape, and the angle thereof is adjusted so that thelaser beam reflected by the VRWM 112 becomes a plane wave. The presentinvention is not limited to transforming one wave into a plane wave, butthe concave wave or convex wave may be transformed into a wavefront witha desired curvature and output. This is the same for each embodimentherein below.

Embodiment 14

A fourteenth embodiment will now be described with reference to FIG. 30.According to the present embodiment, a wavefront compensator 34C, whichcan function as both an angle compensator and a wavefront curvaturecompensator, is constructed by using two convex lenses 114 and 115. Theconvex lens 115 is disposed on a moving stage 117 for adjusting aposition in a direction perpendicular to the optical axis (verticaldirection in FIG. 30). This moving stage 117 is disposed on anothermoving stage 118 for adjusting a position in the optical axis direction.Therefore the convex lens 115 can move in both the axis direction andthe direction perpendicular to the optical axis. The symbol 119indicates a point where light transmitted through the convex lens 114 isfocused (focal point).

FIG. 30A shows a case when the heat load is low. The laser beam of theplane wave transmits through the convex lens 114, and is focused to thefocal position of the convex lens 114. The convex lens 115 is disposedso that the focal position of the convex lens 115 matches with the focalposition of the convex lens 114 on a same optical axis. The lightfocused at the position becomes divergent light, and enters the convexlens 115, then is transformed into a plane wave by the convex lens 115,and is output.

FIG. 30B shows a case when the heat load is high. Due to the influenceof the head load, the entering direction of the laser beam is inclineddiagonally upward, and the laser beam becomes divergent light (convexwavefront). The divergent light focuses more distant from the focalposition of the convex lens 114. Therefore the position of the convexlens 114 is moved in the optical axis direction (horizontal direction inFIG. 30) so that this focal point 119 and the front side focal point ofthe convex lens 115 match. The convex lens 115 is moved in a directionperpendicular to the optical axis (vertical direction in FIG. 30).Thereby the outgoing direction of the laser beam is matched with thereference optical axis direction. The laser beam that passed through theconvex lens 114 enters the convex lens 115 and is transformed into aplane wave, and is output along the reference optical axis.

The present invention is not limited to combining the convex lens 114and the convex lens 115, but the wavefront compensator 34, for executingangle compensation and wavefront curvature compensation, may beconstituted by one convex lens and one concave lens which are combined.

Embodiment 15

A fifteenth embodiment will now be described with reference to FIG. 31.In the present embodiment, a wavefront compensator 34D, which canfunction as both an angle compensator and a wavefront curvaturecompensator, is constructed using a deformable mirror 120 and areflecting mirror 121.

As FIG. 31 shows, the deformable mirror 120 and the reflecting mirror121 are disposed in a Z shape. The shape of the reflecting plane of thedeformable mirror 120 can be variably controlled according to thecontrol signal that is input from the wavefront compensation controller50.

When the laser beam having a deformed wavefront enters the deformablemirror 120, the reflecting plane shape of the deformable mirror 120 isadjusted according to this wavefront that enters. The deformable mirror120 compensates the wavefront of the incoming laser beam to be a planewave, and reflects it. The laser beam compensated to be a plane wave isreflected by the reflecting mirror 121, and is output.

By using the deformable mirror 120, even a wavefront that is notspherical, for example an S-shaped wavefront, can be transformed into aplane wave or desired spherical wave. If the angle is small, thedirection of the laser beam can also be compensated. The direction ofthe laser beam can also be adjusted by controlling the tilt and roll ofthe reflecting mirror 121 and the deformable mirror 120 respectively.This is the same for Embodiment 16.

Embodiment 16

A sixteenth embodiment will now be described with reference to FIG. 32.According to the present embodiment, a deformable mirror 120 andpolarization control are combined to construct a wavefront compensator34E. The wavefront compensator 34E has a deformable mirror 120, beamsplitter 122 and λ/4 plate 123. The SA device 33 can be disposed betweenthe beam splitter 122 and the λ/4 plate 123.

For example, a laser beam of P-polarized light (polarization planeincluding page face) enters a beam splitter 122 in which a film, forseparating P-polarized light and S-polarized light, is disposed. It isassumed that the wavefront of the laser beam is input to the beamsplitter 122 in a state of a plane wave. And it is assumed that thewavefront is deformed to be S-shaped by passing through the SA device 33from the beam splitter 122.

The laser beam that passed through the SA device 33 transmits throughthe λ/4 plate 123, whereby the laser beam is circularly polarized. Thewavefront deformed in an S shape is compensated to be a predeterminedwavefront by the deformable mirror 120, which is adjusted to be anappropriate shape.

The laser beam of which wavefront is compensated transmits through theλ/4 plate 123 again, and is transformed to an S-polarized light. TheS-polarized laser beam transmits through the SA device 33, and thepredetermined wavefront is transformed to be a plane wave. The laserbeam transformed into a plane wave enters the beam splitter 122. TheS-polarized laser beam is reflected by the beam splitter 122, and isoutput as a plane wave. The laser beam could also be output with adesired wavefront profile other than a plane wave by adjusting thesurface shape of the deformable mirror 122.

Embodiment 17

A seventeenth embodiment will now be described with reference to FIG.33. In the present embodiment, a sensor 36B is constructed using awindow 300W. The window 300W has a window substrate 300AW, and a holder300BW for holding the window substrate 300AW. The holder 300BW has awater cooling jacket, which is not illustrated.

The window 300W is disposed on the optical axis of the driver laser beamin an inclined state. A very small part of the laser beam reflected onthe surface of the window 300W enters the optical sensor unit 360 as asample light.

For the window 300W, the window of the amplifier 32 or 35, or the window13 of the EUV chamber 10 can also be used. In this case, it isunnecessary to dispose a window only for receiving the sample light formeasurement, and manufacturing cost can be decreased. The windowsubstrate 300A is constituted by a material having high thermalconductivity that transmits CO₂ laser beam, such as diamond, forexample.

On the plane-parallel window 300W, a small part of a laser beam isreflected on both the front face and rear face, and enters the opticalsensor unit 360 as sample light. Therefore this is not appropriate formeasuring the beam profile. However if the sample light is focused atthe focal position by a focusing lens, the position of the focal imagecan be measured, and the angle (direction) of the laser beam and thedivergent angle of the beam can be measured. The duty and power of thelaser beam line can also be measured without problem.

Embodiment 18

An eighteenth embodiment will now be described with reference to FIG.34. In the present embodiment, light that is reflected by the enteringwindow 3332 (1) of the SA gas cell 330 is measured by the optical sensorunit 360. In order to measure the reflected light, the window 3332 (1)is inclined at angle θ2 from the vertical direction.

According to the present embodiment, a dedicated optical element forsampling the laser beam is not required, but the window 3332 (1) isused, so laser beam loss can be minimized and cost can be decreased.

Embodiment 19

A embodiment will now be described with reference to FIG. 35. Accordingto the present embodiment, a sensor 36C is constructed using beamprofilers 304A and 304B. According to the present embodiment, a profileof the laser beam is detected by a transmitted light beam profiler 304Aof the reflecting mirror 302A, and the transmitted light of thereflecting mirror 302B is detected by the beam profiler 304B. Thewavefront compensator (WFC) 34 is adjusted according to the measurementresult of the beam profiler.

A lens 303A is disposed between the rear face side of the reflectingmirror 302A and the beam profiler 304A. In the same way, a lens 303B isdisposed between the rear face side of the reflecting mirror 302B andthe beam profiler 304B.

The laser beam of the plane wave transmits through the relay opticalsystem 31, and transmits through the SA device 33, whereby the directionof the laser beam and the curvature of the wavefront change. The laserbeam, of which direction and wavefront curvature changed, enters thewavefront compensator (WFC) 34. The wavefront compensator (WFC) 34compensates the wavefront curvature and the angle (direction) of thelaser beam, and outputs the laser beam.

The laser beam compensated by the wavefront compensator (WFC) 34 isreflected by the reflecting mirror 302A and enters the reflecting mirror302B. Meanwhile, a small part of the sample light that transmits throughthe reflecting mirror 302A is transferred onto a two-dimensional sensorof the beam profiler 304A by a transfer lens 303A. By thistwo-dimensional sensor, the beam profile and position of the laser beamare measured.

The measured data from the beam profiler 304A is input to the wavefrontcompensation controller (WFC-C) 50. The wavefront compensationcontroller (WFC-C) 50 sends a control signal to the wavefrontcompensator (WFC) 34 so that the position of the laser beam comes to thereference position.

A small part of the light transmitted through the reflecting mirror302B, on the other hand, is transferred onto the two-dimensional sensorof the beam profiler 304B by the transfer lens 303B. The two-dimensionalsensor measures the beam profile and position of the laser beam.

The data measured by the beam profiler 304B is input to the wavefrontcompensation controller (WFC-C) 50. The wavefront compensationcontroller (WFC-C) 50 outputs a control signal to the actuator 305 foradjusting the angle of the reflecting mirror 302A, and controls theangle of the reflecting mirror 302A so that the position of the laserbeam measured by the beam profiler 304B comes to the reference position.The wavefront compensation controller 50 also sends a control signal tothe WFC so that the laser beam profile becomes a predetermined value, inorder to control the curvature of the wavefront of the laser beam.

In the present embodiment having this configuration, the beam profilers304A and 304B are disposed at the side, where the laser beam transmitsthrough the reflecting mirrors 302A and 302B (rear side of thereflecting mirrors), so the sensor 36C can be compactly constructed.Also influence of the measurement optical system shown in FIG. 37 on thewavefront of the driver laser beam can be minimized.

Embodiment 20

A twentieth embodiment will now be described with reference to FIG. 36.According to the present embodiment, one or more of temperature sensors312 (1) to 312 (4) are disposed in the SA gas cell 330, and thetemperature distribution of the windows 3332 (1) and 3332 (2) isdirectly or indirectly measured.

The first temperature sensor 312 (1) measures the temperature of themixed gas that flows into the flow space 3334 in the SA gas cell 330.The first temperature sensor 312 (1) can be disposed in the middle ofthe supply pipeline 333 (1) or at the inlet connected to the supplypipeline 333 (1), for example. The second temperature sensor 312 (2)measures the temperature of the mixed gas that is exhausted from theflow space 3334. The second temperature sensor 312 (2) can be disposedin the middle of the exhaust pipeline 333 (2) or at the outlet connectedto the exhaust pipeline 333 (2), for example.

The heat load determination unit 314 can calculate calories Q from thedifference between the temperature Tin at the inlet side and thetemperature Tout at the output side (Q=k(Tout−Tin): k is a proportionalconstant). Calories Q correspond to the quantity of light absorbed bythe mixed gas. Based on these calories Q, the state of the heat load ofthe SA gas cell 330 can be calculated. The heat load determination unit314 outputs the calculation result to the wavefront compensationcontroller 50.

The third temperature sensor 312 (3) detects the temperature around theedge of the output window 3332 (1), and outputs it to the heat loaddetermination unit 314. The third temperature sensor 312 (3) isconstructed as a contact type temperature sensor, such as asemiconductor temperature sensor or thermocouple. The heat loaddetermination unit 314 can measure the state of the heat load of the SAgas cell 330 based on this temperature.

The fourth temperature sensor 312 (4) is a non-contact type temperaturesensor, and measures the surface temperature of the input window 3332(1) at a distant location. The fourth temperature sensor 312 (4) is, forexample, a radiation thermometer. Based on the temperature and thetemperature distribution detected by the fourth temperature sensor 312(4), the heat load determination unit 314 measures the state of the heatload of the SA gas cell 330.

Embodiment 21

A twenty first embodiment will now be described with reference to FIG.37. According to the present embodiment, the input window and the outputwindow are constructed as a common window, and a reflection opticalsystem 3335, for reflecting the incoming laser beam, is disposed in theflow space 3334.

The SA gas cell 330D of the SA device 33A has a single window 3332 onone side. This single window 3332 is a diamond window, for example, andhas both functions of the input window and reflecting window. A highreflection mirror 3335 is disposed in the SA gas cell 330D, so as toface the window 3332. The high reflection mirror 3335 is installed sothat the laser beam enters and is reflected at a small incident angle.The installation angle of the high reflection mirror 3335 may be set toa value whereby the incoming beam of the laser and the reflecting beamof the laser are separated outside the saturable absorption cell. Thehigh reflection mirror 3335 is a mirror that reflects the laser beam athigh reflectance. In the high reflection mirror 3335, a coolingmechanism (not illustrated) using cooling water is disposed so that themirror is not deformed by the heat of the laser beam.

The laser beam enters the flow space 3334 via the window 3332, and isreflected by the high reflection mirror 3335. The reflected laser beampasses through the window 3332, which was passed when entering, and isoutput to the outside of the SA gas cell 330D.

According to the present embodiment, as described above, the laser beamis reflected back to the SA gas cell 330D, so one window 3332 can beused as the input window and output window. Hence the SA gas cell 330Dcan be manufactured by using only one diamond window, which isexpensive, the manufacturing cost can be decreased.

Embodiment 22

A twenty second embodiment will now be described with reference to FIG.38. In the SA device 33B of the present embodiment, two high reflectionmirrors 3335 (1) and 3335 (2), with a 45 degrees incident angle, areused in the SA gas cell 330E. A cooling mechanism (not illustrated) isalso disposed in each high reflection mirror 3335 (1) and 3335 (2).

Each high reflection mirror 3335 (1) and 3335 (2) are disposed inclined45 degrees respectively from the optical axis of the laser beam, so asto reflect the laser beam that enters from the window 3332 toward thewindow 3332. The present embodiment having this configuration alsoexhibits the same effects as the twenty first embodiment.

The present invention is not limited to an individual embodiment. Thoseskilled in the art could add and change in various ways within the scopeof the present invention. Configurations appropriately combining theabove embodiments shall be included in the scope of the presentinvention.

1. An extreme ultraviolet light source device for generating extremeultraviolet light, the device comprising: a target material supply unitfor supplying target material into a chamber; a laser oscillator foroutputting a pulse laser beam; at least two amplifiers for amplifyingthe laser beam that is output from the laser oscillator; a focusingoptical system for irradiating the laser beam after the amplificationonto the target material by focusing the laser beam, which is amplifiedby the amplifier and is output, to a predetermined position in thechamber; and a saturable absorber device, disposed on an optical pathbetween the laser oscillator and the predetermined position, forabsorbing at least a laser beam having light intensity not greater thana predetermined value and suppressing laser transmission, and thesaturable absorber device comprising: a saturable absorber cell having amain body unit that has a flow space where the saturable absorber flows,an inlet for letting the saturable absorber enter the flow space, anoutlet for letting the saturable absorber out from the flow space, and awindow for passing the output laser beam from the laser oscillator tothe flow space; a pipeline for connecting the inlet and the outlet; atransport unit, disposed in the middle of the pipeline, for transportingthe saturable absorber that flows out of the outlet, so as to flow intothe flow space via the inlet; and a temperature adjustment unit,disposed in the middle of the pipeline, for adjusting temperature of thesaturable absorber transported by the transporting unit.
 2. The extremeultraviolet light source device according to claim 1, wherein thesaturable absorber cell comprises: an input window that is disposed onan input side where the output laser beam of the laser oscillatorenters; an output window that is disposed on an output side where theoutput laser beam of the laser oscillator is output; the flow spaceformed between the windows; the inlet; and the outlet.
 3. The extremeultraviolet light source device according to claim 1, wherein the inletand the output are disposed so that the flow of the saturable absorberin the flow space becomes approximately symmetric with respect to anoptical axis of the laser beam that passes through the flow space. 4.The extreme ultraviolet light source device according to claim 2,wherein the inlet and the outlet are disposed so that the flow of thesaturable absorber in the flow space becomes approximately symmetricwith respect to an optical axis of the laser beam that passes betweenthe windows.
 5. The extreme ultraviolet light source device according toclaim 2, wherein the inlet or the outlet is disposed near each of thewindows, so that the saturable absorber flows on an inner face side ofeach of the windows and the saturable absorber moves along the opticalaxis of the laser beam between the windows.
 6. The extreme ultravioletlight source device according to claim 2, wherein the inlet is disposedon the input window side and the output window side respectively, andthe outlet is disposed between the windows.
 7. The extreme ultravioletlight source device according to claim 2, wherein the outlet is disposedon the input window side and the output window side respectively, andthe inlet is disposed approximately at the center between the windows.8. The extreme ultraviolet light source device according to claim 6,wherein the inlet is disposed inclined toward the center of the innerface of each of the windows.
 9. The extreme ultraviolet light sourcedevice according to claim 7, wherein the outlet is disposed inclinedtoward the center of the inner face of each of the windows.
 10. Theextreme ultraviolet light source device according to claim 2, whereineach of the windows is formed to be circular, a plurality of the inletsare disposed on the input window side and the output window siderespectively, so as to be axially symmetric with respect to the opticalaxis of the laser beam that passes between the windows, each of theinlets disposed on the input window side is disposed in parallel with atangential line direction of the input window, and each of the inletsdisposed on the output window side is disposed in parallel with atangential line direction of the output window, and a plurality of theoutlets are disposed between the windows so as to be axially symmetricwith respect to the optical axis of the laser beam.
 11. The extremeultraviolet light source device according to claim 2, wherein each ofthe windows is formed to be circular, a plurality of the outlets aredisposed on the input window side and the output window siderespectively, so as to be symmetric with respect to the laser beampassing between the windows, each of the outlets disposed on the inputwindow side is disposed in parallel with a tangential line direction ofthe input window, and each of the outlets disposed on the output windowside is disposed in parallel with a tangential line direction of theoutput window, and a plurality of the inlets are disposed between thewindows so as to be symmetric with respect to the laser beam.
 12. Theextreme ultraviolet light source device according to claim 2, whereineach of the window is formed to be circular, a plurality of the outletsare disposed on the input window side and the output window siderespectively, so as to be axially symmetric with respect to the opticalaxis of the laser beam that passes between the windows, and a pluralityof the inlets are disposed between the windows so as to be axiallysymmetric with respect to the optical axis of the laser beam, and a flowcontrol member, in which a plurality of flow holes for letting thesaturable absorber flow are formed, is disposed between an outercircumference of inner face side of each of the windows and each of theoutlets.
 13. The extreme ultraviolet light source device according toclaim 12, wherein the flow control member comprises: a tubular memberwhich is disposed coaxially in each of the windows, and one edge ofwhich is disposed on the inner face side of each of the windows, andwhich has a the flow hole individually; and a ring-shaped collar portionthat covers an area between the other edge of the tubular member and aninner wall portion of the flow space, so that the saturable absorberdoes not flow into each of the outlets from an area other than each ofthe flow holes.
 14. The extreme ultraviolet light source deviceaccording to claim 2, wherein a plurality of the saturable absorbercells are disposed, such that a flow direction of the saturationabsorber in one saturable absorber cell and a flow direction of thesaturated absorber in the other saturable absorber cell are oppositefrom each other.
 15. The extreme ultraviolet light source deviceaccording to claim 2, wherein the input window and the output window areconstituted by a common window, and a reflection optical system forreflecting the output laser beam of the laser oscillator that entersfrom the common window and letting the laser beam output from the commonwindow is disposed in the flow space.
 16. The extreme ultraviolet lightsource device according to claim 1, further comprising a wavefrontcompensation device for compensating a wavefront of the laser beam thatpasses the saturable absorber device.
 17. The extreme ultraviolet lightsource device according to claim 16, wherein the wavefront compensationdevice comprises: a wavefront measurement unit for directly orindirectly measuring a direction and a wavefront profile of the laserbeam; a wavefront compensation unit for compensating the direction andthe wavefront profile of the laser beam to be a predetermined directionand a predetermined wavefront profile; and a wavefront control unit foroperating the wavefront compensation unit based on a measurement resultfrom the wavefront measurement unit.
 18. The extreme ultraviolet lightsource device according to claim 1, further comprising: a control unitfor controlling the transport unit and the temperature adjustment unit.19. The extreme ultraviolet light source device according to claim 1,wherein the temperature adjustment unit is constituted by a heatexhausting device for exhausting heat absorbed by the saturableabsorber.
 20. The extreme ultraviolet light source device according toclaim 1, wherein the temperature adjustment unit is constituted by aheat exhausting device for exhausting heat absorbed by the saturableabsorber and a control device for controlling the temperature of thesaturable absorber.
 21. An extreme ultraviolet light source device forgenerating extreme ultraviolet light, the device comprising: a targetmaterial supply unit for supplying target material into a chamber; alaser oscillator for outputting a pulse laser beam; at least twoamplifiers for amplifying the laser beam that is output from the laseroscillator; a focusing optical system for irradiating the laser beamafter the amplification onto the target material by focusing the laserbeam, which is amplified by the amplifier and is output, to apredetermined position in the chamber; and a saturable absorber device,disposed on an optical path between the laser oscillator and thepredetermined position, for absorbing at least a laser beam having alight intensity not greater than a predetermined value and suppressinglaser beam transmission, and the saturable absorber device comprising asaturable absorber cell having a main body unit that has a flow spacewhere the saturable absorber flows, an inlet for letting the saturableabsorber enter the flow space, an outlet for letting the saturableabsorber out from the flow space, and a window constituted by diamondsfor passing the output laser beam from the laser oscillator to the flowspace.
 22. A pulse laser device, comprising: a laser oscillator foroutputting a pulse laser beam; at least two amplifiers for amplifyingthe laser beam that is output from the laser oscillator; and a saturableabsorber device, disposed on an optical path between the laseroscillator and the amplifier, or on an optical path between theamplifiers, for absorbing at least a laser beam having light intensitynot greater than a predetermined value and suppressing laser beamtransmission, and the saturable absorber device comprising: a saturableabsorber cell having a main body unit that has a flow space where thesaturable absorber flows, an inlet for letting the saturable absorberenter the flow space, an outlet for letting the saturable absorber outfrom the flow space, and a window for passing the output laser beam fromthe laser oscillator to the flow space; a pipeline for connecting theinlet and the outlet; a transport unit, disposed in the middle of thepipeline, for transporting the saturable absorber that flows out of theoutlet, so as to flow into the flow space via the inlet; and atemperature adjustment unit, disposed in the middle of the pipeline, foradjusting temperature of the saturable absorber transported by thetransporting unit.
 23. The pulse laser device according to claim 22,wherein the temperature adjustment unit is constituted by a heatexhausting device for exhausting heat absorbed by the saturableabsorber.
 24. The pulse laser device according to claim 22, wherein thetemperature adjustment unit is constituted by a heat exhausting devicefor exhausting heat absorbed by the saturable absorber and a controldevice for controlling the temperature of the saturable absorber.
 25. Alaser device, comprising: a laser oscillator for outputting a pulselaser beam; at least two amplifiers for amplifying the laser beam thatis output from the laser oscillator; and a saturable absorber device,disposed on an optical path between the laser oscillator and theamplifier, or on an optical path between the amplifiers, for absorbingat least a laser beam having light intensity not greater than apredetermined value and suppressing laser beam transmission, and thesaturable absorber device comprising: a saturable absorber cell having amain body unit that has a flow space where the saturable absorber flows,an inlet for letting the saturable absorber enter the flow space, anoutlet for letting the saturable absorber out from the flow space, and awindow constituted by diamonds for passing the output laser beam fromthe laser oscillator to the flow space.
 26. A method for controlling asaturable absorber that is used for an extreme ultraviolet light sourcedevice that generates extreme ultraviolet light, the method comprisingthe steps of: circulating the saturable absorber in a saturable absorbercell in which a laser beam for generating the extreme ultraviolettransmits; maintaining temperature of the saturable absorber to becirculated at a predetermined temperature; and letting the saturableabsorber flow in the saturable absorber cell, so that temperature ofdistribution of one or a plurality of windows of the saturable absorbercell becomes approximately symmetric with respect to an optical axis ofthe laser beam.